My idea of how ayando(ayanoxbudo) would work is that somehow, Yan-chan and budo become friends, may it be through a task, her being in the martial arts club, or a combination of those and/or more. Through that friendship, budo, with his gusto personality and genuine friendliness, becomes ayano's best friend. Ayano just kind of accepts this and plays along, acting as his friend, because having the trust of a martial arts master could be a real advantage.

However, it grows worse for him and better for her, because as more students disapear, and as he develops a crush, budo becomes more paranoid of a killer, and becomes very concerned for his best friend's safety. And when people dare accuse her, he gets very, very angry. He knows yan-chan better than anyone, and she wouldn't hurt anyone, except in self defense!

So, its better for her in the way that he can not fathom the idea that she could possibly hurt anyone, and therefore, defends her from a good amount of reputation loss, and let's her stay in the martial arts club, if she's in it, regardless of what the others say. He is the leader, after all.

But it's worse for him in the way that if the others see how obvious it is that Ayano is killing or hurting people, budo's reputation and status will drop. And once his usefulness has worn off, Ayano can kill him...

...or show him mercy, and keep up the act to the very end. If it's even an act by that point.