So not a huge difference from Canon Octavian at this stage but there will be more differences- especially one scene towards the end of this fic.
Replies:
Weirdhead: Percy just has zero respect for him so that'll be fun- and obviously since it's Mars he doesn't actually know her. Also fun.
Guest: That would be spoilers if I told you- and yeah, basically, Reyna knows that Percy could probably do a hell of a lot of damage before they could stop her so seeing her get so freaked out would not be comforting at all.
Undeath9087: Honestly I love Bianca in this- she feels so guilty and she's desperate to help- and she is not as closed off as Nico was in the books so she's not exactly good at hiding things the way he was.
The Official Clarisse: Oh he'll suffer just as he deserves, just not quite yet. But remember this is a Percy who is in a lot of ways more powerful than in canon and she hates people like him so that'll be fun. And yes Bianca and Reyna will still be happening. At this stage they've both sorta got crushes they just haven't realised it yet and Bianca is in denial.

Percy had to consider Hazel's words for a long moment before she mentally translated the Latin words into English. "Jupiter... the best and the greatest, did I get that right?"

"Yeah." Hazel nodded and Percy snickered to herself.

"Sounds like he's a bit of a showy drama king-" that felt right- though how she knew that she wasn't sure. After all it wasn't like she could have met him, "Anyway, what's Neptunes title then?" Percy tilted her head, "The coolest and most awesome?"

"Um, not quite." Hazel gestured to a small blue building the size of a toolshed. A cobweb-covered trident was nailed above the door.

Percy peeked inside. On a small altar sat a bowl with three dried-up, moldy apples.

Her heart sank. "Popular place."

"I'm sorry, Percy," Hazel said. "It's just…Romans were always scared of the sea. They only used ships if they had to. Even in modern times, having a child of Neptune around has always been a bad omen. The last time one joined the legion… well, it was 1906, when Camp Jupiter was located across the bay in San Francisco. There was this huge earthquake—"

"You're telling me a child of Neptune caused that?"

"So they say." Hazel looked apologetic. "Anyway… Romans fear Neptune, but they don't love him much." Percy stared at the cobwebs on the trident. It made her chest ache. Her father didn't deserve this she was sure of it- the lack of respect.

And what would it mean for her? Would her new campmates fear her? She didn't want that- sure she knew she was powerful, probably scarily so to some people but she didn't enjoy hurting people or causing massive damage- or she didn't think she did.

Still, standing in her fathers alter she did feel something stirring inside of her. She'd been warned against praying to her father by Lupa, she'd said that certain gods might interfere if she tried- but- maybe now she could. In his temple- surely he'd hear her if she prayed to him.

She reached into her backpack and dug out the last bit of food from her trip- a stale bagel that she was pretty sure she'd break her teeth on if she tried to bite into it.
It wasn't much but she set it down on the alter and took a deep breath. "Hey dad." her voice was quiet- she felt stupid- almost vulnerable with Hazel watching her but- "I don't really know what's going on here. I don't- I don't have my memories so if you could fix that it would be great. I just- tell me- tell me what to do. Please.
Help me. I- who am I? I want to remember. Please."

She blinked back tears, and she felt Hazel's hand on her shoulder again- supportive this time rather than trying to pull her back.

"It'll be okay. You're here now. You're one of us."

She felt awkward- Hazel was younger than her, she shouldn't be having to comfort her. But still, she was glad the other girl was there.

Above them, thunder rumbled. Red lightning lit up the hill."Octavian's almost done," Hazel said. "Let's go."

Compared to Neptune's tool shed, Jupiter's temple was definitely optimus and maximus.

The marble floor was etched with fancy mosaics and Latin inscriptions. Sixty feet above, the domed ceiling sparkled gold. The whole temple was open to the wind.
In the center stood a marble altar, where a kid in a toga was doing some sort of ritual in front of a massive golden statue of the big dude himself: Jupiter the sky god, dressed in a silk XXXL purple toga, holding a lightning bolt.

"It doesn't look like that," Percy muttered.

"What?" Hazel asked.

"The master bolt," Percy said.

"What are you talking about?"

"I—" Percy frowned. For a second, she'd thought he remembered something solid Now it was gone. "Nothing, I guess."

The kid at the altar raised his hands. More red lightning flashed in the sky, shaking the temple. Then he put his hands down, and the rumbling stopped. The clouds turned from gray to white and broke apart.

A pretty impressive trick, considering the kid didn't look like much. He was tall and skinny, with straw-colored hair, oversized jeans, a baggy T-shirt, and a drooping toga. He looked like a scarecrow wearing a bed sheet.

"What's he doing?" Percy murmured, resisting the urge to turn and run.

The guy in the toga turned. He had a crooked smile and a slightly crazy look in his eyes, like he'd just been playing an intense video game. In one hand he held a knife. In the other hand was something like a dead animal. That didn't make him look any less crazy.

"Percy," Hazel said, "this is Octavian."

"The graecus!" Octavian announced. "How interesting."

Percy twitched again- she didn't like the way the way his eyes had focused on her. "Uh, hi." she swallowed hard, her throat dry. "Are you killing small animals?"

Octavian looked at the fuzzy thing in his hand and laughed. "No, no. Once upon a time, yes. We used to read the will of the gods by examining animal guts—chickens, goats, that sort of thing. Nowadays, we use these."

He tossed the fuzzy thing to Percy. It was a disembowelled teddy bear. Then Percy noticed that there was a whole pile of mutilated stuffed animals at the foot of Jupiter's statue.

"Seriously?" Percy asked incredulously.

Octavian stepped off the dais. He was probably about eighteen, but so skinny and sickly pale, he could've passed for younger. At first he looked harmless, but as he got closer, Percy wasn't so sure. Octavian's eyes glittered with harsh curiosity as they drifted up and down her- she felt like a piece of meat on display. And his eyes glittered with something that- it made her stomach twist uncomfortably.

Octavian narrowed his eyes. "You seem nervous."

"I-" Percy swallowed hard, "Why would I be?"

"Very good question." Octavian's sly smirk set her even more on edge than she already was.

"I think you remind me of someone. I just don't remember."

"Possibly my namesake, Octavian—Augustus Caesar. Everyone says I bear a remarkable resemblance."

Percy didn't think that was it, but she couldn't pin down the memory. It wasn't even a physical appearance thing she didn't think, just that look in his eyes when he looked at her "Why did you call me 'the Greek'?"

"I saw it in the auguries." Octavian waved his knife at the pile of stuffing on the altar. "The message said: The Greek has arrived. Or possibly: The goose has cried. I'm thinking the first interpretation is correct. You seek to join the legion?"

Hazel spoke for her. She told Octavian everything that had happened since they met at the tunnel—the gorgons, the fight at the river, the appearance of Juno, their conversation with Reyna.

When she mentioned Juno, Octavian looked surprised.

"Juno," he mused. "We call her Juno Moneta. Juno the Warner. She appears in times of crisis, to counsel Rome about great threats."

He glanced at Percy, as if to say: like mysterious Greeks, for instance.

"I hear the Feast of Fortuna is this week," Percy said. "The gorgons warned there'd be an invasion on that day. Did you see that in your stuffing?"

"Sadly, no." Octavian sighed. "The will of the gods is hard to discern. And these days, my vision is even darker."

"Don't you have…I don't know," Percy said, "an oracle or something?"

"An oracle!" Octavian smiled. "What a cute idea. No, I'm afraid we're fresh out of oracles. Now, if we'd gone questing for the Sibylline books, like I recommended—"

"The Siba-what?" Percy asked.

"Books of prophecy," Hazel said, "which Octavian is obsessed with. Romans used to consult them when disasters happened. Most people believe they burned up when Rome fell."

"Some people believe that," Octavian corrected. "Unfortunately our present leadership won't authorize a quest to look for them—"

"Because Reyna isn't stupid," Hazel said.

"—so we have only a few remaining scraps from the books," Octavian continued. "A few mysterious predictions, like these. I do my humble best but- well, it's not easy."

He nodded to the inscriptions on the marble floor. Percy stared at the lines of words, not really expecting to understand them.

Instead she almost choked as her eyes focused on one of them. "That one!" she pointed it, her eyes wide as she read it out loud. "Seven half-bloods shall answer the call, to storm or fire, the world must fall, to hope to win worlds must unite-"

"Yes, yes." Octavian finished it without looking: "Held together by the twice liveds light, an oath to keep despite a final breath, and foes bear arms to the Doors of Death." she could almost hear a voice- a young voice, female, telling her the prophecy.

"I- I know that one." Percy gasped out, her face deathly pale, "It's important- I know it-" she was shaking- shaking badly- a whole body thing, her heart was pounding in her chest.

Octavian arched an eyebrow. "Of course it's important. We call it the Prophecy of Seven, but it's several thousand years old. We don't know what it means. Every time someone tries to interpret it…Well, Hazel can tell you. Bad things happen. You say you know it, that is very interesting-"

Hazel glared at him. "Just read the augury for Percy. Can she join the legion or not?"

Percy could almost see Octavian's mind working, calculating whether or not Percy would be useful. He held out his hand for Percy's backpack. "That's a beautiful specimen. May I?"

Percy didn't understand what he meant, but Octavian snatched the Bargain Mart panda pillow that was sticking out of the top of her pack. It was just a silly stuffed toy, but Percy had carried it a long way. She didn't want this creepy guy cutting it up- "Wait!"

Octavian had the pillow pet in his hands, and he paused, raising an eyebrow, still gripping his knife. "What is it?"

"You can't cut that up!" at his raised eyebrow Percy's face flushed.

"Why not?"

"I-" Percy felt like a dumb kid, "It's mine."

Octavian stared at her for a long moment, a thoughtful look on his face, "What would you suggest I do instead?"

"... I don't know." Percy muttered the words, they left a sour taste in her mouth, "I just- I don't have much of my own and- I don't want that destroyed too."

Octavian's eyes glinted as he stared at her for another long moment before tossing the panda back at her, and Percy caught it and hugged it close, her eyes widening as he moved towards a pile of stuffed toys, "It may not give me as good of a reading but I'm sure that this will be good enough. I wouldn't want to take away a source of comfort." the smile he gave her was almost genuine- warm and reassuring- but- Percy felt like she'd been on the receiving end of smiles like that before- it was the sort of smile she imagined a wolf would give a sheep to lure them in closer.

"Uh- thanks."

She winced as he slashed a stuffed teddy's belly open and poured its stuffing all over the altar. He tossed the teddy carcass aside and muttered a few words over the fluff, and he turned, a big smile on his face.

"Good news! Percy may join the legion. We'll assign her a cohort at evening muster. Tell Reyna that I approve." his eyes flicked up and down Percy again- and she was painfully aware of how she probably looked. "Yes, yes I think I certainly approve."

She also felt Hazel relaxing next to her, "Uh, great. Thank you Octavian. Come on Percy, let's go."

"Oh, and Hazel-" Octavian's voice was smooth, like melted chocolate suddenly, sweet and inviting. "I'm happy to welcome Percy into the legion. But when the election for praetor comes up, I hope you'll remember—"

"Jason isn't dead," Hazel snapped. "You're the augur. You're supposed to be looking for him!"

"Oh, I am!" Octavian pointed at the pile of gutted stuffed animals. "I consult the gods every day! Alas, after eight months, I've found nothing. Of course, I'm still looking. But if Jason doesn't return by the Feast of Fortuna, we must act. We can't have a power vacuum any longer. I hope you'll support me for praetor. It would mean so much to me."

Hazel clenched her fists. "Me. Support. You?"

Octavian took off his toga, setting it and his knife on the altar. Percy noticed seven lines on Octavian's arm—seven years of camp, Percy guessed. Octavian's mark was a harp, the symbol of Apollo.

"After all," Octavian told Hazel, "I might be able to help you. It would be a shame if those awful rumors about you kept circulating…or, gods forbid, if they turned out to be true."