Nightmare-verse Background: Keigo was born into an average family, consisting of himself, his parents and his older sister Mizuho. He was a very smart boy and played well with other children--despite being a little shy at first--, much to his parents delight. Sometime in kindergarten, he met Kurosaki Ichigo and the two of them became fast friends, mostly due to Ichigo's friendly nature. It was a rare occasion to see one without the other when they were little; Ichigo's confidence helped Keigo get over his shyness and they meshed well when they played together. This continued for a few years, until when they were both about 8 or 9. Ichigo's mother, Masaki, had died and not long after that, Ichigo became reclusive and quiet, a complete 180 from how he was before. Keigo didn't quite understand why he became like that, being young and still unfamiliar with the concept of death, but he still tried to get Ichigo to smile and play like they used to, with little success.
When they were 10, Ichigo's family moved away and Keigo had little contact with him after that, despite sworn promises to become pen-pals. The letters stopped and eventually, Keigo moved on with his life.
But, not without a little change of his own. While Keigo had become somewhat outgoing thanks to Ichigo, after Ichigo left, Keigo was alone again. His parents had very busy schedules and while they were loving and kind people, they just didn't have the time to spend with their children, often leaving Mizuho and Keigo in the hands of babysitters, or each other. This weighed heavily on Keigo, hating to be alone, especially with Mizuho, as his sister was overbearing and sometimes cruel to him. This left Keigo to keep to himself most of the time to avoid her wrath, busying himself with doodling, reading comic books and playing video games.
The loneliness soon took a toll on his grades, which were previously rather impressive, and Keigo became lazier and lazier. He felt that without anyone around, he had no one to impress, and so he lost interest in keeping up appearances, as far as his grades. It wasn't until middle school did he make another friend in Kojima Mizuiro, but by that time, Keigo had become very set in his ways.
So much so, that, while Mizuiro managed to break Keigo out of his shell once more, his grades just plummeted further. And Mizuiro was a horrible enabler; every chance they had, he would call Keigo up to go hang out, mostly to hit on girls at the mall.
And so, through the years of secondary school, Keigo became a first-class flirt, hitting on his female classmates left and right and without shame, usually ending in his face getting pummeled in by the more jealous boys in his classes. He had grown used to the beatings and even developed a sort of tone to use with the aggressors in his classes, shrewd and cowardly. He was often used as a gofer, and Keigo had little shame or pride left to openly protest.
After secondary school, Keigo's bad grades had left him with little options left as far as high schools he could attend. Mizuiro had already been set with his studies, proverbially leaving Keigo in the dust. It was only by his sister's nagging did he bother to use that latent intelligence to get himself into Nightmare High.
But, once he got in, Keigo just fell back into the same old habits, slacking on studies and rarely, if ever, doing homework. His sister occasionally threatens him to do work and he does just enough to scrape by, but he won't go that extra mile for an A+.
Keigo's love for swimming has also gotten his sister to force him into actually being in a club, namely the swim team. Keigo doesn't enjoy having his free time eaten up by something lame like an after-school club, but for fear of angering his sister, he does it anyway. Keigo looks on it as a way to go swimming without making the long drive to a beach and doesn't take the activity as seriously as he probably should.
Even without Mizuiro around to enable him, Keigo is still very social and outspoken, willing to talk to just about anyone, provided they aren't assholes or rude. There is also one other little clause in Keigo's socializing and that is the class of nerds. If Keigo suspects someone to be a nerd, chances are he won't take the time of day to talk to them, let alone be seen with them unless absolutely necessary. Even if that was the case, Keigo would complain and complain the entire time, anyway.