Aura. The manifestation of one's soul. That was the basic description that Summer had offered him when Shirou first asked about the topic.

The very concept itself was something that the young Magus had trouble accepting. For one thing, the soul was largely considered a very complicated topic of study, at least for most Magi. Heck, one of the True Magics was all about the materialization of the soul, and while Aura may have its uses, it was far from the level of True Magic.

Still, from how Summer had described it, Aura was definitely a boon that most Magi would doubtlessly kill for. For the people of this world, Aura was a powerful advantage with a number of different uses. Summer had unlocked his in order to save his life, taking advantage of Aura's life-saving capabilities in order to accelerate his natural healing process.

Shirou later proved this by self-inflicting an injury (just a small cut on his finger). In a few seconds, the wound had fully healed. He was honestly impressed.

Unfortunately, Summer had forewarned him that he should not overly rely on his Aura in order to survive from fatal injuries. For one thing, yesterday's event was a bit of an exception to the rule. Since his Aura was newly unlocked, it worked overdrive in order to save his life. Shirou won't be so lucky the next time. Aura's actual capabilities were not so potent.

Well, it wasn't like Shirou was complaining. At the very least, he was doubtlessly more durable than before. For someone with a dream like his, such an advantage could not be ignored.

"You seem to have a lot of it." Summer had stated, sounding quite curious. "And there's something else about it too…"

"Something else?" He parroted in confusion. "What sort of something else?"

"It was…" Her words trailed off. The huntress recalled back to that endless world of steel and smog. It had only been for a mere moment, but she knew that the sight of that desolate, lonely world would forever haunt her memories.

"What is it?" Shirou urged.

"No, it's nothing." She shook her head. "Forget I said anything. Now, come on kiddo. It's already day out. Let's get going."

She did not wait for a response before she walked out of the room, her twin long sword dangling from her hip. Shirou hurriedly chased after her.

Like that, Emiya Shirou's journey through the Grimmlands kicked off.

A few days passed.

The duo traveled during the day and rested at night. Without exception, the environment that surrounded the forest was perpetually gloomy. Fortunately, it could do nothing to lessen Shirou's amiable mood. How could he not be happy? He had learned so much in the past few days! Had he been a traditional Magi, then surely, he would have died from sheer joy by now.

One of the first things that Summer had taught him were things pertaining to their current situation.

They were currently stuck in a place known as the Grimmlands. As its name implies, it was a place choked full of Grimm. In fact, there was not a single human settlement anywhere on this continent. In order to reach civilization, the duo needed to first find the southern coastline. Shirou felt quite daunted by the task. Mainly, he had no clue how far in-land they were, so who knows how long the trip would take.

Unlike him, Summer was not bothered by the distance. If anything, she's remained rather positive these past few days. It was a different sort of positivity than Shirou's, the type acquired through sheer confidence in one's own abilities. That's what it probably was. Summer was an accomplished fighter. With her Aura up and about, she was confident in her ability to escape the Grimmlands, regardless of how far they had to travel to do so.

That was another thing that he noticed about Aura. It was a renewable resource. All it took was a night's rest, or even a few hours of downtime, for Aura to fill back up to optimal levels. In a way, it was kind of like Od, the raw energy found within all living things. Maybe they were one and the same?

Thoughts for later.

Summer mainly did the heavy lifting when it came to combating the various threats throughout their travels. Shirou offered to help, but she shot him down every time. Unfortunately, even if he was given the opportunity, there was little help he could really provide, at least to a veteran huntress like Summer Rose.

Instead, Shirou decided to practice with his aura as they traveled.

Summer had previously explained to him that Aura was predominantly used for protection against harm. In essence, a force field. Shirou noted how similar this application was to Reinforcement. The main difference between the two was that Aura was a lot more… instinctual. Or maybe emotional was the better word? In either case, her only advice regarding the subject was to dig deep inside him and imagine a mysterious force coating his skin.

It was vague and completely different from the systematic approach of Magecraft.

If he had to compare the two, then Aura was a system better suited for warriors, while Magecraft was a system for scholars. Aura was like manipulating a limb to perform an action, while Magecraft was like solving a math problem. In both cases, Shirou preferred the latter.

Still, the appeal of an automatic force field was not lost on him.

The fact that Aura was largely instinctual was a major advantage, as it meant that Shirou did not have to worry about concentrating in order to keep his Aura active while in the midst of combat. Unfortunately, the process of keeping Aura active as a shield was something that the body must first recognize, then memorize. It was sort of like martial arts; he needed to repeat the same action indefinitely until the action itself becomes instinct.

In between his training, Shirou also watched for any changes to his body.

Physically, there were no changes— at least, not any that he had observed. There might be some gradual changes in the future as a result of his body constantly being in contact with Aura, but that was something for the future him to figure out. As of right now, everything about his body was the same as it had been before.

Well, that's not entirely true. Something did change, just not with his physical body. It was a change that affected the metaphysical space of his body, where things such as conceptual weapons and spiritual organs were located. Basically, the same space where his magic circuits were found. While his circuits were predominantly located on his arms, this change affected his entire body. A form of energy suffused throughout every inch of his personal space.

If he had to describe it, then it was as if his entire body was turned into one giant circuit; a vessel capable of containing Magical Energy.

From the sheer size alone, the capacity of this new 'vessel' far outpaced the capacity of his magic circuits. It was more than ten times as large. Of course, this isn't to say that one was better than the other. For one, density was a major factor that simply could not be ignored. How dense was the energy in his body compared to the energy in his circuits? Unfortunately, Shirou did not have the current ability to find out, at least not without extensive research, something that he couldn't really do while on the move.

Then, there was also the matter of efficiency. How did Aura (for what else could it be) compare to Magical Energy? How much of the stuff was required to perform a specific action, and how would it compare if he did the same action but used the Magical Energy found within his circuits instead?

Last but not least was capability. What was Aura capable of? Was it just a clump of raw energy capable of performing minor feats such as self-enhancement and personalized force fields, or could it be used in a manner similar to Magical Energy? Could he use it to somehow reenact mysteries?

Clearly, Shirou had a lot on his plate. The Magus in him wanted nothing more than to dive head-first into research, a sentiment that honestly took him by surprise. Ever since he first learned of Magecraft, Shirou never really considered himself much of a Magus. He was more of a Spellcaster, like his dad. Someone who used Magecraft as a means to an end. Apparently, Shirou was a lot more of a Magus than he gave himself credit for.

Unfortunately, there was little actual research he could do while traveling through the Grimmlands. At most, he could speculate and form theories, but that was pretty much it.

There was one thing though, something that he discovered by simply asking a question.

"Can Aura be increased?"

In response, Summer Rose said, "Of course it can. People get stronger when they train. The same thing goes for Aura. The more you use it, the more it will grow, kind of like a muscle. Actually, it has a lot to do with the human body. The more you train your body, the better it gets at generating and holding Aura. "Your body is a temple," that sort of thing. So, don't skimp out on strength training, alright? It might not be as fun as fighting Grimm, but it's still a pretty important part of becoming a huntsman. Er, I guess it would be hero for you. Speaking of which, we should probably get you a weapon. You can't be a hero without a personal weapon, after all. And no, a kitchen knife is not a weapon. Although learning how to fight without a weapon could work too. You know, my husband fights barehanded—"

At some point, Shirou stopped listening. Summer Rose had a habit of rambling on and on. Not that it mattered though. He already got his answer and what he heard, utterly blew his mind.

Aura can be increased.

To him, it was a monumental discovery. It was common knowledge amongst Magi that Magic Circuits were a static thing. A person's natural circuit count was set in stone. Other than through implantation of Magic Crests, the number of circuits available to a Magus doesn't increase. That was just how it worked. In other words, there was a capstone to a Magus's potential via their circuit count and the overall quality of their circuits, both of which were set in stone.

To him, Aura was an absolute game-changer.

After his hype had settled down, the discovery left him with but a single thought: Was Aura a completely separate system from Magic Circuits, or was it possible to somehow get the two to interact? For example, could he use his Aura in order to fuel his Magecraft?

The mere notion of it was crazy. Personally, Shirou had his doubts. After all, if he could really combine the two systems, it would completely overhaul Shirou's potential as a Magus. It would transform him from a third-rate spellcaster into someone rivaling the great Magus families of the Clock Tower, all of whom possessed lineages spanning hundreds, if not thousands of years.

It was simply too good to be true. Still, Shirou was hopeful.

One reason for his optimism was the smell. Summer Rose smelled.

His father had explained it once to him, but most Magi can detect Magical energy through a form of synesthesia. For Shirou, he could detect the activation of Magical Energy via scent. Aside from Kiritsugu, Shirou had never met other Magi before. His only experience with this ability of his was with his father. The old man's Magecraft smelled of gun-powder and ash.

Summer Rose possessed a similar scent. Hers did not smell of gunpowder; instead, it was more along the lines of a flower bed. Also, her scent was a lot lighter than Kiritsugu's Magecraft and constantly wafted all around her, sort of like body odor or perfume.

Naturally, there were differences between the two, but one thing was clear: Summer Rose smelled faintly of Magecraft. To Shirou, it was proof enough that there might be some merit to his crazy idea.

"Shirou, keep your head down. We don't want any of them to spot us." Summer Rose chastised the boy with a disapproving frown.

They were currently crouched behind a tipped-over log.

"Does it even matter?" He asked. "I thought you said these things could sense us coming? Won't they already know that we're here?"

"They can, but it never hurts to be careful!" She responded back heatedly. "And don't raise your voice. They'll hear you… probably."

"I think it's already too late for that." The young magus pointed in front of him, towards the nearby stream of water.

Summer followed his finger, only to grimace in frustration.

"Crap baskets! They spotted us!" She narrowed her eyes and stared down at the opposition. It was a pack of bipedal, frog-like Grimm with what looked to be a dining plate on top of their heads. To Shirou, they looked a lot like Japanese Kappa.

The pack was nearly two dozen strong. It wasn't a whole lot (not to Summer, at least), but the huntress was getting really tired of Grimm. There were just too many of them on this damn continent! She breathed out a frustrated sigh and unsheathed her weapons.

"Stay here, Shirou." She instructed. "Wait patiently."

Before he could respond, the veteran huntress darted off to confront the Grimm. Shirou watched her go before he let out a sigh of his own. Once upon a time, he would have happily offered his assistance. Unfortunately, as he was now, he'd be little more than deadweight. Unironically, it was more helpful for him to do nothing than to actually get involved in the fighting.

It was a disheartening thought.

Shirou let out another sigh as he reflected back on his time here.

Three weeks have passed since his arrival to Remnant. Admittedly, a bit of his initial enthusiasm has died down since then.

Aside from Summer's verbal instructions, Shirou hasn't really gotten much training done. To his disappointment, his mentor was unable to fully commit to the training. As their only combatant, Summer needed to maintain her Aura reserves. Unfortunately, that meant things like full-contact sparring were out of the menu.

While on this continent, the Grimm were her first priority. Shirou understood this, so he tried not to let it bother him too much.

That was another thing too. The monsters made of shadows; they were actually called Grimm.

Humanity's greatest bane. These creatures dotted the world like an infestation of pests. They were driving this world's version of humanity to the very brink of extinction. Also, they could apparently sense negative emotion and unlocked aura.

Shirou wondered briefly if these things were a byproduct of some sort of Magic gone wrong, or maybe the spawn of some type of divine entity. Were they perhaps a type of Phantasmal Beast? Unfortunately, the young magus has never met a Phantasmal Beast before, so he lacked a point of reference.

In either case, they encountered quite a number of Grimm throughout their journey. The number has surely surpassed the thousands by now. Without exception, Summer Rose mercilessly slaughtered them all. For those that ran on foot, they met the steel of her blades, while those that flew up above were sniped down with her bow.

That was another thing too. Apparently, her weapons came with a function called mecha-shifting. When he had analyzed her weapons, he already had an inkling of this feature, but seeing it in person was a completely different experience.

In her case, Summer's twin swords could combine to form a giant compound bow. The bow shot out these arrows that appeared to be composed entirely out of hardened light. When Shirou asked how she created the light arrows (at first, he presumed it to be a form of Aura manipulation), she had explained how the arrows were created via the hard-light dust reserves located within the spine of her blades.

Shirou's understanding of the process was a bit iffy, but it still impressed him, nonetheless.

Summer was fast with the bow and her accuracy was near perfect from up to 600 meters away, which was impressive, considering the fact that most modern bows from Earth had a maximum range of around 400 meters. Accuracy with said weapon was even lower.

In either case, Shirou mainly learned through verbal instruction from Summer. To be honest, it wasn't much. Shirou quickly learned that Aura training mostly revolved around increasing a person's natural aura reserves. Basically, strength training and combat practice. In terms of actually manipulating aura, there really wasn't much available. Aura was more about instinctive use, such as learning how to keep your Aura up while in combat, or learning how to strengthen your limbs with Aura.

To a Magus like him, it was basic, beginner stuff.

From this, Shirou learned that Aura simply was not meant to be used in the way that Magical Energy was used to enact mysteries. It was too unwieldy. Aura was like a giant pool of energy, kind of like a battery or a generator. Unfortunately, it was just that, energy. As useful as energy could be, it was kind of useless on its own.

Aura lacked an interface. Unlike with Magical Energy, it did not have its own version of Magical Circuits to help with the whole Magecraft thing. Without a circuit-equivalent, Aura was limited to the basic stuff, like protection from harm or self-enhancement.

What's more, Shirou learned that Aura was not very cost-effective. For example, the energy required to block an attack with Aura was five times greater than the energy required to block an equivalent attack with Reinforcement.

In essence, Aura was unwieldy and overly wasteful. For huntsmen like Summer, it didn't matter much. After all, it didn't matter how wasteful Aura was when they've got a whole bunch of it at their disposal. For Shirou, it was a weird experience. Back on Earth, he had learned how to be conservative with his energy in order to stretch out his use of Magecraft. Here, it was the exact opposite. If anything, wasting one's Aura was actually beneficial, as Aura exhaustion could also act as a form of training.

Despite all of his new discoveries, there was still one question that lingered in Shirou's mind: could he substitute Magical Energy for Aura and use it to fuel his Magecraft?

Even after three weeks on Remnant, he had yet to come to an answer. Since he was constantly on the move, Shirou did not really have much of an opportunity to experiment or do research. At most, he could speculate during his free time.

Shirou shifted his focus back to the fight at hand. Or rather, to the end of the fight.

He caught a glimpse of a white shadow rushing towards the last remaining Grimm. With a motion that Shirou had trouble following, Summer Rose bisected the Kappa from the head all the way down to its crotch. As the two parts began rapidly dissipating, Shirou stood up from his hiding spot and walked over to the huntress.

Upon seeing him, Summer immediately darted over to him and began to raise a fuss. "Are you okay? None of the Grimm got past me, did they? You made sure to stay properly hidden, right?"

In the face of her incessant worrying, Shirou felt an uncharacteristic jolt of annoyance. Shirou waved his arms in the air and said, "I'm fine. Summer, I'm fine, really."

She acted like a doting mother. Granted, he really should have expected this. After all, this tiny adult was actually the mother of two children. Shirou would have had trouble believing such a thing, were it not for the way she sometimes smothered him with affection and concern.

Shirou honestly did not know what to do with the attention. He never really had a mother before. If he did, then he did not remember her. The closest thing that he had to a mother was Taiga, his surrogate older sister, and current guardian. Even then, Taiga wasn't the motherly sort. If anything, he took care of her, not the other way around.

Shirou believed that Summer was projecting. Apparently, her children were just a few years younger than him. Shirou probably reminded him of them. And while he sympathized with the woman, her attitude sometimes got pretty annoying.

"We should set up for the night." He suddenly spoke up, just in time to interrupt another tirade from the white-cloaked huntress. "It's already pretty late."

Shirou looked up at the concrete sky. From his experience with this place, it should be somewhere around six to seven PM. Nightfall would occur in a little bit less than an hour.

Summer nodded in agreement. "Right. Some rest would be good for you. After all, Shirou's still a growing boy."

The young magus let out a withering sigh. He would have refuted her statement, but he knew well enough that it would have been a fruitless effort. The duo walked deeper into the forest. After a short while, they stumbled upon the husk of a giant tree that was as wide as a vertical-standing shipping container.

Summer quickly proceeded to hollow out the trunk of the tree with her swords. She was preparing it as their shelter for the night. Meanwhile, Shirou set up a fire and began cooking dinner (fish that he had managed to scrounge up from the stream).

After dinner, Shirou conducted his nightly routine.

It was only during the nights that Shirou was allowed time to practice his forms and stances. All of this was under Summer's guidance, which Shirou was thankful for. Prior to his arrival on Remnant, he did not really have much in the way of combat education. This was because Kiritsugu did not have a whole lot of time left to teach Shirou (or in general), so he prioritized Magecraft and neglected other aspects. In Kiritsugu's mind, combat was something that Shirou could learn on his own time, but Magecraft was a highly guarded art that not many would be willing to teach.

Shirou learned more about fighting in three weeks with Summer than he did in three years all on his own. Still, he was not fully satisfied. Shirou would have liked to do some live sparring.

Of course, the boy had no illusions about winning against a veteran huntress like Summer. From what he had glimpsed from her weapons, Summer Rose was something of a prodigy in combat. Still, that did not mean he would back down. As his father had once taught him, even defeat could serve as a learning opportunity.

Combat training usually lasted for about an hour or two. Afterward, the pair would take turns sleeping.

It was during the dead of night when Summer slept idly and the silence of the forest was loudest, that inklings of dread and worry crept into his heart.

While Shirou was certainly overjoyed at the prospect of finally learning how to be a proper hero, he still couldn't help but harbor concerns. He wanted to go home. There were things back on Earth that he still needed to do, people that he wanted to see. The image of his surrogate older sister popped to mind. He had been gone for so long, surely Taiga was worried sick. Then, there was his other sister, the one that he had yet to meet. At this rate, would he even be able to meet her?

Shirou was scared. Worry gnawed on him like a dog to a bone. Despite his background as a Magus, despite his lofty dreams and even loftier promises, at the end of the day, Emiya Shirou was still just a vulnerable thirteen-year-old boy, a boy who had lost his father not even a year ago.

It was a lot to take in. It was also why he spent most of his nights trying to come up with a way to get back to Earth.

Time passed.

Summer continued to fret over him like a doting mother, while Shirou continued his training with Aura and further internalized his worries about finding a way back home.

As the pair drew nearer to the coastline, Summer's mood gradually lightened. At the same time, the number of attacking Grim gradually decreased as well. From a thousand to a hundred and from a hundred to a couple of dozen. At this point, seeing more than ten Grimm in a day was considered a lot.

The huntress's mood had improved so much, that she finally agreed to one of the things that Shirou had been constantly pestering her about ever since the start of their journey.

She finally agreed to a spar.