A young man was riding his bike to school. He had black hair with brown eyes. His uniform consisted of a black jacket with white outlines. Beneath it was a white shirt with a black ribbon. The young man's name was Katsuro Sumeragi, a third-year student going to Kuoh Academy. The school he goes to has quite a name in town. For one, it started as an all-girls school until five years when it turned into a co-ed one. Despite the change, the ratio of guys to girls in the school was 1/4. Second, the school has a good number of foreign-exchange students. It has the highest number of them in the country.

Katsuro locked his bike in the bollard before entering the main building. When he reached the second floor, Katsuro gave a glance outside the window. A few sports clubs are active in the morning, doing warm-ups in the fields or quick matches in the gym or courts.

"There you are, man!" a male student shouted. Katsuro spotted the Perverted Trio, the most infamous group in the school. He said, "We were looking for you yesterday but you disappeared!"

"Yeah, you missed out on some pretty good angles!" the individual with glasses said. "It turns out that the tennis captain's bust increased by 5cm."

"Holy shit, really?!" a brunet male said. "If she keeps it up, she may not be so flat-chested by graduation!"

Katsuro winced at the trio's comment. They're known for talking about the boobs and sex out in the open. Worse of all is that they're repeated offenders of peeping in the lockers. While Katsuro finds their actions repulsive, he does envy them a bit.

"Must be nice to have friends," Katsuro muttered before looking back at the fields. "What am I even doing here?"

The young man sighed, reflecting on his time at the school. It was an uneventful time for him. He was more or less a background character that never did anything significant. Katsuro wondered if this was going to be another uneventful year.

Katsuro's day proceeded as normal. When lunch came, he slipped away from the large crowd of students. He made his way towards the back of the school, in the area between the gym and an old building. It was a nice, quiet spot near the back of the school. As he approached his normal spot, Katsuro spotted a black, circular item on the bench. He picked it up and inspected it.

"Congratulations," a man spoke from behind Katsuro. He turned around and spotted a man in a brown coat with a grey hood and scarf. He held a book and said, "According to this book, today will be the day you begin your ascension to greatness. But, be wary of the dangers in the park."

"What're you talking about?" Katsuro questioned.

"In due time, everything will be revealed," the hooded man said. He said, "That device in your hand is the key to your survival."

Katsuro glanced down at the circular device in his hand before looking back. The man in front of him disappeared.

"I must've been hallucinating…" Katsuro muttered.

Katsuro decided to pocket the circular device for now. He sat down and enjoyed his lunch. In the distance, through the window of the old building, he spotted a classmate of his. A red-haired beauty stood by the window. The two made brief eye contact before closing the curtains. Her name was Rias Gremory, a foreigner from Europe. Not much is known about her except that she's the president of the Occult Research Club.

"It's hard to believe like her can be a part of such a disturbing club," Katsuro thought. "I wonder what creepy stuff happens once the curtain closes."

The rest of the day proceeded normally. Once school ended, Katsuro headed straight home. His home was a two-floor building. The first floor was an electronic and appliance store. It was a small business that his uncle ran. In the back was where the kitchen and dining room were. The second floor was the living quarters. Katsuro took his bike through the back of the store, which was where the kitchen and dining room were.

"I'm back!" Katsuro announced.

"Welcome back," Hatsu Sumeragi, Katsuro's aunt, greeted. She was prepping food for later tonight. Katsuro entered the front of the store. His uncle, Hotaru Sumeragi, was busy repairing a radio.

"Ah, you're back," Hotaru said. "How's school?"

"Same as usual," Katsuro replied.

"When am I going to hear you say, "Oh, I made a friend?" or "I've asked a girl out"? Hotaru questioned.

Katsuro sighed and sighed, "Eventually."

"Eventually? You're in your third year of high school and I saw you done nothing in those two years. High school is such an important period in your life. You disregarding it isn't going to help you in the future."

"Right," Katsuro nodded.

"I'm serious," Hotaru said as he looked up.

"I'll do fine on my own. It worked out for the last two years, I'm sure I'll do fine," Katsuro said.

"Have you ever thought about what you want to do after high school?"

"I'm not sure yet, that's what I'm trying to figure out myself," Katsuro replied.

"Well, you have less than a year to figure it out. Although the sooner you figure it out the better."

Katsuro was dressed in a track jacket and sweat pants, ready to go jogging. On his desk, was the circular object he found earlier. He picked it up and inspected it. Earlier today a person warned him that today is his "ascension" and that this device is the key to it. Katsuro wondered if what happened was a hallucination or not. Either way, it wouldn't hurt for him to bring this along. If it was useless he can throw it out anytime. That aside, it seemed that the park was the place where it was going to happen. Conveniently it's a part of his normal jogging route.

Katsuro walked downstairs and walked past the kitchen. He announced, "I'm going to go out for a quick jog."

"Alright," Mrs. Sumeragi said. "Dinner will be ready by the time you back."

While he wasn't athletic in any way, Katsuro wanted to stay in shape at the very least. Every weekday he would do jogging around dusk when it was cooler. As Katsuro reached the park, he pulled out the device and looked around.

"Maybe it was all in my head…" Katsuro muttered.

"Hahaha… prey…" a menacing cackle came from the trees. Katsuro turned around and a masked man immediately lunged at him. He was dressed in Victorian-style clothing and wielded a scythe. He had bar wings protruding from his back. "Quick one, aren't you?"

"What're you supposed to be?!" Katsuro shouted, trying to remain composed. He took a few steps back to give himself distance from the crazed man.

"I'm the Devil known as Edmond! I'm here to rip you, humans, apart!" the Devil shouted as he lunged toward Katsuro. He barely dodged the swing of the scythe.

Katsuro immediately bolts out of the scene. Edmond cackled as he flew up to the sky. Katsuro ran out of the park and into a large, empty street. Suddenly, the attacker from the park swept down from the skies. His loud cackle revealed the direction of the attack. It allowed enough time for Katsuro to face him and dodge the attack. Yet, the scythe managed to cut his arm. Luckily for Katsuro, the cut wasn't deep.

On top of a building not too far, Rias Gremory stood with three other students from Kuoh Academy. One was a third-year student named Akeno Himejima. She had purple eyes with long, black hair tied to a ponytail. The second student was Yuuto Kiba, a popular student from the second year. He had short, blonde hair with blue eyes. The third student was Koneko Toujou, a first-year student with white hair and yellow eyes. She was the shortest out of everyone there.

"It looks like we arrived in time to stop this Stray from killing a civilian," Yuuto said.

"Oh my, Rias, isn't that boy from our class?" Akeno asked.

Rias nodded and said, "He is."

"We should interfere before he kills him," Koneko said as she tightened the gloves around her hand.

"Right, let's get this over with-" Rias was interrupted by a voice from behind.

"I request that you don't interfere with my king's transformation," the voice spoke. Rias and her group turned around to see the hooded man that talked to Katsuro earlier. "According to this book, this is the moment where begins his journey to become king."

"King?" Rias muttered. "Which faction do you or Katsuro belong to?"

The hooded man smirked as Katsuro cried out in the background. The group turned around to see Katsuro with a new wound on his wrist. He looked at the device, wondering why it isn't doing anything. Suddenly it glowed and something formed around his waist. Edmond descended to get a better look at what was going on in front of him. Rias and her group also focused their attention on Katsuro.

Ziku Driver!

A belt formed around his waist. The buckle was a rotating unit with two slots on the side. It had a timer display in the center.

"What's going on?" Katsuro questioned. The watch-like device transformed. It now has a white frame with a picture of a masked man with the year "2018" written below.

"Your ascension…" the hooded man appeared as he walked out of cover. Rias and the others turned around to see that he teleported somehow below.

"My what?" Katsuro asked.

"If you use that Ridewatch, you'll transform into Kamen Rider Zi-O," the man explained. "It shall mark your ascension to kingship."

"Why would I want to become a king?" Katsuro asked, thinking the concept was ridiculous.

"Because it is your destiny. Besides, if you don't transform, how else would you defend against the Stray Devil?"

The Devil scoffed and shouted, "You really think you have the strength to overpower a Devil?!"

"We will see," the hooded man said. He turned to Katsuro, "My king, I humbly request you transform to show this Devil his place."

"A-Alright…" Katsuro said. He turned the frame of the Ridewatch, revealing the full portrait of the masked man. He pressed the button on top.

Zi-O!

Katsuro placed the watch-like device on the right slot before pressing the button on top. It caused the buckle to tilt and a construct of a clock appeared behind Katsuro.

"It can't be… a Kamen Rider…" Rias muttered.

"Transform!" Katsuro shouted as he rotated the belt in a circle.

Rider Time!

Several metallic bands formed around Katsuro and spun around. The belt had Zi-O written on the belt and the words "Rider" in katakana appeared on the clock behind him.

A leather suit with bits of armor replaced his tracksuit and sweats. His suit was black with pink highlights on the hips. He also sported a black shoulder that had a pink and purple outline. Katsuro also wore a mask that has a face that resembles a watch. Uneven clock hands formed the antenna of the mask. A metallic band runs down the center of his chest and back to make it look like a watch.

Kamen Rider! Zi-O!

The pink letters flew out of the clock and slammed into Katsuro's face. It formed the visor of the mask.

"Rejoice!" the hooded man proclaimed. "Inheriting the power of all Riders! Kamen Rider Zi-O! Today shall be the day he starts his ascension to becoming king of all!"

"All Riders...?" Rias wondered, continuing her observation of the events. She thought, "What does that imply?"

"A Kamen Rider? Tch, I've heard about you people. You think you're tough shit because you killed a few monsters," Edmond scoffed. Rias' group was bewildered by the spectacle.

"What should we do, Rias?" Yuuto questioned.

"Leave them be. I want to see where this goes," Rias said.

Back below, Kamen Rider Zi-O clenched his fist and raised them. He proclaimed, "I feel like I can do anything with this power!"

Zi-O charged at Edmond, who in return lunged at the Rider while pulling back an attack. Just as Edmond swung his scythe, Zi-O quickly punched him in the chest. Edmond was surprised by the attack and he quickly prevented his fall to the ground. Zi-O then threw a kick that Edmond blocked. However, it staggered him back. As Zi-O fought against the Devil, Rias and her group remained spectators.

"So is he a Devil or a human?" Yuuto questioned.

"Human without a doubt, and not only that but a Kamen Rider as well," Rias replied.

"What's a Kamen Rider?" Koneko asked.

"It's a nickname given to any human with a transformation device. We know that a lot of people with varied ideologies adopted the mantle of Kamen Rider. This means that while they are good guys for most of the time, there are evil ones."

Back at the fight between Zi-O and Edmond, the Stray Devil slashed the Kamen Rider across the chest. Sparks flew out of Zi-O's chest as he was sent flying back. An object began to materialize out of Zi-O's belt. When he grabbed it, it formed a sword.

Zikan Girade!

Edmond gritted his teeth as he swung in front of another attack. Zi-O quickly blocked it and parried it with a quick stab to the chest. The Kamen Rider then swung his sword downward but Edmond quickly jumped back and into the air.

"You Riders are tough but are still mortal humans at the end of the day!" Edmond shouted as he swept down for an attack. Zi-O blocked the attack but the force knocked him down. As he got up, he noticed a slot in the shape of the watch device on his belt. He pulled the watch device from his belt and plugged it into the sword.

Finish time!

Zi-O jumped into the air to close the distance between him and Edmond.

Zi-O GiriGiri Slash!

With a single strike, Zi-O managed to slice Edmond in half from the waist. An after-image of a clock formed as the sword cuts through the Devil. Both halves of Edmond blew up as Zi-O landed on his feet.

"So this is the strength of a Kamen Rider…" Rias muttered.

"Not bad, he handled that Stray Devil with ease compared to most beginners," Yuuto commented.

"I never imagined someone like Katsuro would be so capable in a fight," Akeno added.

"Hm, let's go," Rias said as she turned around.

"Don't you want to introduce yourself to him? He could be a worthy ally to us," Yuuto said.

"Us Devils don't interfere with human affairs. Whenever a new Kamen Rider appears, it's a sign that humanity is being threatened," Rias said. "We let these Kamen Riders act on their own since they never interfered with our affairs."

"But he fought against a Stray," Koneko pointed out.

"Only in self-defense," Rias said. She suddenly felt distressed, "Oh no…"

"What is it?" Akeno questioned.

"A Fallen Angel is attacking our newest Pawn."

"He must think he's the Stray we're ordered to hunt down," Koneko said.

"Come on, we can worry about the Kamen Rider later…"

Rias formed a magical circle to teleport everyone out of the scene. Back below, Katsuro was ecstatic about his recent victory. He felt a rush he never felt before.

"Awesome! I can't believe I managed to do all of that!" Zi-O shouted. Once he looked around, the hooded man was gone. As Katsuro calmed down, he began to wonder who was it that he just fought and if there were more of them.

"Oh, well, if there's more of them, I'll fight them with his new power," Zi-O proclaimed. The Kamen Rider's stomach growled and he realized that dinner was ready. He said, "Oh, crap, mom's going to get worried.

Katsuro took off the belt, which transformed back to his civilian clothing. He began sprinting back home.

It's been a few days since Kamen Rider Zi-O appeared, and, to Rias' knowledge, has not appeared ever since. The crimson-haired Devil reported that a Kamen Rider dealt with the Stray Devil. The Archduke made note of the new Kamen Rider but didn't proceed with an investigation.

The Underworld does not meddle with human affairs unless it involves them. Kamen Riders always acted as independent agents responding to various threats that could end the world. Their journeys saw little divine intervention however they did get the attention of the supernatural world. There were a few close calls that could've ended the world. Some of these events include like a virus outbreak from a virtual disease or an invasion from an alternate dimension. What some of these Riders accomplished was comparable to the mythological heroes of the past.

While understanding the current affairs with the Kamen Rider would be in her best interest, Rias was occupied with her Pawn. She recently acquired him when she found him after he was attacked and nearly killed by a Fallen Angel. As she walked to the old building after school, Rias realized something was wrong. Outside of the building were the rest of her Peerage.

"Our defenses have been breached," Akeno said.

"How can someone be strong enough to break through?" Rias questioned.

"They weren't destroyed. They were dismantled," Koneko pointed out. Rias felt it, or more specifically the lack of magic present. If it was destroyed there should've been a disperse of demonic energy. However, there's no trace of it left.

"We're currently waiting for your command," Yuuto said.

"I'll do what I can!" Issei enthusiastically proclaimed.

"Alright, Yuuto, Koneko, take point. Everyone else, follow them," Rias commanded. She added, "Our first objective is to secure the main room."

"Right," everyone said in unison. Yuuto and Kiba charged in. As a Knight, Yuuto was faster and broke through the door to the main room. Koneko soon followed.

A lone woman with black hair and in white garbs was present in the room. She was surprised by the loud entry but didn't show any sign of fear or distress. Rias and the others soon followed. Since the woman didn't have any weapon or a spell ready, Rias didn't see her as a threat.

"Stand down," Rias ordered. "She shows no threat towards us, for now. So tell me who you are and why are you here?"

The woman said, "I'm Tsukuyomi, and I come with a warning from the future."

Author's Note:

Welcome to Kamen Rider Zi-O: A Fate of Two Kings. A crossover between the titular Kamen Rider series and High School DxD.

The first thing I'd like to talk about is the main protagonist: Katsuro Sumeragi. Now, it's not like I don't like Sougo Tokiwa but it's just I'd rather have my own spin on the titular Rider. Katsuro will be a completely different character than Sougo and the first obvious divergence is that Katsuro doesn't want to be a king. He's a more down-to-earth character who's been trusted into the chaotic world of Rider and Devils.

Now, this will be an interesting story to write. I'm basically dumping 20 seasons of Kamen Rider into the world of DxD. In this story, the Underworld is aware of Kamen Riders' activities but largely ignores them. Some apocalyptic scenarios like the Helheim invasion from Gaim or the Bugster outbreak from Ex-aid are significant enough that everyone in the Underworld would know about them. I want to make the relationship between the Riders and the Underworld to be something interesting.

I've watched a good chunk of the Heisei Rider seasons, at least the ones relevant for the first few arcs of Zi-O. If you're curious, I've watched Ex-Aid, Den-O, Faiz, Fourze, Gaim, Drive, Build, Ryuki, OOO, and Kabuto. I obviously watched Zi-O however it was pretty dumb of me to watch an anniversary season as my third Rider season. My reasoning was that it could highlight some Rider seasons and I'd watch it based on which episodes intrigued me the most. Basically, I thought it would be like Gokaiger but I was dead wrong.

Finally, I want to talk about the focus characters of this series. The main focus would be Katsuro Sumeragi/Kamen Rider Zi-O and Rias Gremory. Y'know, the two kings from vastly different worlds and the titular characters. Issei will take a backseat but all of the development and growth will still happen.

By the time you're reading this, I've written chapters 3-6 and will release them weekly in batch orders as I have other stories I want to work on. This chapter is short because it acts as a quick origin story for Zi-O. The next chapter will be longer.

Anyway leave a favorite and follow if you enjoyed the story. Please leave constructive criticism, good or bad, in the reviews.

Up next: Message from the Future