The Oni of SAO (Sword Art Online x OC)

Mamoru Kayaba was the younger brother of the creator of SAO Akihiko Kayaba. He was always enormous compared to the other kids his age for as long as he could remember. The reason for this was unknown to him, but his giant size made it hard for him to fit in. As a result, he didn't make that many friends, causing him to struggle to talk to people. That being the case, Mamuro formed a dependency on his brother Akihiko Kayaba. The Kayaba brothers were inseparable, doing nearly everything together.

Neither of them was very social growing up, preferring to confide in each other for companionship. Both brothers had restless imaginations, with a recurring theme being a floating castle high above the clouds. As the brothers grew, Akihiko showed stellar performance in his studies for someone his age, as did Mamoru with his physical capabilities. Once the prodigies left high school, they immediately got flooded with invitations from organizations in their respective fields.

While Mamoru joined several wrestling organizations and even represented Japan in the Olympic games, Akihiko joined a major corporation where he began work on the death game of Sword Art Online. Mamoru even helped in its creation by acting out the movement and attacks of the various monsters, plus a few of his suggestions made it into the game. On the day of its release, Mamoru was allowed access a few hours earlier by his brother to explore and gain a few levels before everyone else.

But that was when he realized just what his brother did. Now trapped within the game with death looming over him, Mamoru tries his best to clear the game himself as atonement for his help in the players' death sentence.

Although, it's not all bad, as here he will meet people he could call friends, and no matter what happens, he will protect his new companions with the ferocity of a demon.

Just how far is he willing to go to protect them?