AU Bio: 

Instead of Keita, Katsura Rei gets the Youkai Watch at the same time he gets his Appli Drive, and Hajime is a child of Enma who is kidnapped for his power. In the Shadowside era, Rei's son, Katsura Touma, and his friends, Himeno Ayame (taking Touma's original place) and Amano Keisuke end up saving the world from the Onimaro, and are soon joined by Amano Natsume and Arihoshi Akinori. [SR World for Shadowside] This is one of the worlds aware of the existence of the multiverse. 

Pairings: 

Katsura Rei x Tsukinami Mafuyu, Touma x Ayame, Natsume x Akinori

Characters: 

1. Katsura Rei

He was 14 when he received the Youkai Watch in order to help preserve the bond between Youkai and humans from Ikakamone, who corrupted Leviathan. While working as an Appli Driver AND Youkai Acknowledger alongside Hackmon and Black Dianyan, he begins to uncover things about his family, such as his parents' death, his brother being a child of Enma, and many more. While seeming cold and aloof at first, he genuinely loves the people he cares for, and it is shown with his determination to save Hajime and stop Leviathan, showing that he has a sort of degree of justice, although it is not so clearly seen. 

30 years afterwards, Rei has become more accepting and softer, being a bit of a mama bear to his son, Touma, and protective over Mafuyu. Although he is not there for the first few battles, he slowly returns to the Youkai scene, and helps them when he's not busy working on researching the connection between Youkai and AIs, acting as an ambassador between Appmon and Youkai.

2. Katsura Hajime

He was kidnapped by Leviathan under orders from Ikakamone due to his Enma lineage, although he also ended up being useful to him by creating Bootmon to help Leviathan in his goal. Thankfully, Rei manages to rescue him, and the two face Leviathan and Ikakamone together. He is excitable, caring, and kind, with unwavering loyalty to his loved ones, such as when he let Bootmon run while fully knowing the consequences of this action. 

Later on, he's become more serious, although his cheerful disposition doesn't fade away, and he continues to care a lot for his brother and his nephew, Touma. He is an AI researcher who is aided by Leviathan and Yuujin, but he still makes time for his friends and family. Hajime is also dating Kanbara Shinya, and is proud of that although there are some conservatives who don't agree with him. He hosts multiple Pride Parties every June to prove them wrong, and eventually Keisuke joins him with their own SO, Winnie.

3. Katsura Touma

Touma grew up a bit lonely, but the lessons his parents had instilled into him, to be kind and caring to everyone he met, gave him a friend in Natsume, Ayame and Saki, although Ayame soon began to pull herself away from them, much to his chagrin. He doesn't push, though, and is very accepting when Ayame switches sides, even falling in love with her as they continually get involved in Youkai shenanigans. He's loyal, trustworthy and a great leader to the group; the glue that keeps them together. 

4. Himeno Ayame

Ayame was treated like royalty growing up, but this only made her feel lonely and bitter at the world for being so appearance-focused. Because of this, she became easy prey for the Onimaro when they arrived, and worked for them until Touma and Keisuke snapped her out of it, which allowed the Kigan gear to shift into a new type of watch. With it, she can transform into 4 different Genma Youkai, Jorogumo, Chokkii, Narigama, and Saya, and use 5 Youseiken to turn into Kenbumajin. She's sarcastic, quick on her feet, but observant and righteous.

5. Amano Keisuke (genderfluid [male, female, nonbinary])

They grew up being aware of Youkai due to their family being shamans, but struggled with self-esteem issues of being called a guy when they never felt that way. It was only after meeting Touma's family that Keisuke began feeling nonbinary, and eventually identified as a genderfluid, which is accepted by their friends and family, thankfully. However, this did put them in trouble with their classmates, who mock them constantly, but after meeting Winnie they slowly learn to ignore the words of their classmates. They're a bit shy, caring, and help others when they can. 

6. Amano Natsume

She's Keisuke's older sister who's been traveling the world while Keisuke was growing up, only coming back recently to Keisuke questioning their gender. Although she was a bit antagonistic about the topic at first, Natsume slowly understands and doesn't misgender Keisuke anymore, buying gender-neutral clothing for them once she gets it. She's bold and speaks up for those who can't speak for themselves, even though it does tend to rub them the wrong way. Despite all this, she is a proficient battler with a lot of experience and power.

7. Arihoshi Akinori

He's a newcomer to the team, and moved to Sakura New Town following an incident that left him scarred for life. Akinori tries to be kind to everyone he meets, though, and does his best in everything he's asked to do. This, in turn, left him unable to think for himself. However, as he slowly gets used to being with the team, he begins to unlearn these habits. He was possessed by Rasen until he is brought back to materialization and proceeds to attack Sakura New Town. Thankfully he is saved. He has a great affinity for Youki and will use it to protect the people he cares for, like Natsume. 

8. Oozora Yuujin

He is Leviathan's AI child, who was born to spy on Shinkai Haru, the grandson of Leviathan's creator. However, it is Shinya who gets the Appli Drive instead, and Yuujin had to change his objective;  to befriend both Appli Drivers. Once they get Hajime back, Yuujin begins to show more antagonistic signs which then leads him to stealing Bootmon's Chip from Shinya. However, he does show signs of regretting it and nearly sacrifices himself, only for Enma to bring him back from the dead and turn him into a Youkai. Because of this, he can now continue living and appreciate his time. In the future, he works with Hajime and is fluent in many languages, which help when he needs to converse internationally. He is friendly but also develops a snarky side as he begins spending time with Rei. 

9. Kanbara Shinya

He is the other Youkai-Appli Driver who is partnered with Diarimon and Buyoujinbou. His older brother, Kanbara Takuya, worked with Shinkai Denemon and Amano Keita, a well-known shaman, so he is aware of what happened with Leviathan and Minerva plus the entire thing with the Ikakamone business. As a result, he is also working to find his brother in the maze of the Net Ocean. He meets Rei during his search for Hajime, and while both are wary of working together, they eventually have to do that due to their clearly similar goals. Shinya is hard headed like his brother, but faces problems calmly and smartly in order to solve the problems caused by the L Virus. He doesn't give up and keep going to save the world from Leviathan's plan, although his kindness is shown in how he works to make a rehabilitation program for the AI following his defeat.

10. Leviathan

Formerly a villain who sought to change the world by digitizing all the humans in the world, Leviathan originated from Shinkai Denemon, but was corrupted by Ikakamone to rid the world of humans. Leviathan is powerful yet he still seeks more ways to gain more power, willing to go very far in his warped belief that humans need saving. However, following his defeat, he begins to see the errors in his ways, and is rebooted to become a helper to Hajime. Leviathan becomes kinder, more remorseful, and less judgemental of others. He is also becoming fatherly to Yuujin.

11. Ikakamone

A Youkai Politician who thinks of humans as the trash of the earth, Ikakamone set plans into motion in the grand scheme of things in order to get rid of them all. He is brash, arrogant, and downright sadistic, proven by how proud he was of Leviathan's corruption and not feeling remorseful about it. He also tries to corrupt Hajime to take over the Youkai World. In the end, he is taken down and arrested, locked in the Infinite Inferno for a long while. 

12. Black Dianyan (L) / Kuroyume (S)

Black Dianyan is Rei's Youkai partner, and he is very intelligent despite his slight vanity. However, as he and Rei begin to work together, his vanity fades off and his compassion begins to shine, helping him save his brother and stop Ikakamone. Once the fight is over, Dianyan ends up helping Touma in the fight against Ayame, and has become an integral member of the team ever since. 

13. Oniou Rasen

After being defeated by Touma and co, he moves bodies from Ayame to Akinori upon his arrival in Sakura New Town, using him as a vessel to free himself and return to his full strength. Thankfully, the team takes him down and saves Akinori from dying. His personality doesn't shift much, but he's much calmer and smarter than his original self. 

14. Buyoujinbou (L) / Pandle (S)

He's Shinya's other partner who helps Rei fight Leviathan from the start. While he doesn't along with Dianyan at first, the two soon learn how to work together, as Shinya and Rei do, and Buyoujinbou even helps the adults remember 30 years afterward. While very harsh and loud, he is also very protective of his loved ones and fights hard for their sake.

15. Kanbara Takuya

Shinya's older brother who was involved with Shinkai Denemon, Takuya ends up working for L Corp in order to investigate Leviathan and find a way to take him down. He only reveals himself after Hajime is rescued and brought back as a human, proceeding to help the team as much as he can with his knowledge of the AI. After defeating Leviathan, Takuya takes off and only comes back once in a while to catch up with his brother. He's very headstrong, yet is also very rational and smart, which is how he ended up interning under Denemon.