Koshi is a caring boy, who is protective of his friends and alarmingly resilient. He was stabbed twice, buried alive, and still managed to crawl out of it. He is rather honest about his feelings and wastes no time telling the truth when it's necessary. He is quite the do-gooder, attempting to stop Yukari and Miho from killing anybody else in Liar Liar 2: Pants on Fire, and even going as far as trying to stab Yukari as she tried to kill her boyfriend in Liar Liar. However, in the Liar Liar 2 Demo, Koshi has proven to be manipulative of others. Koshi is described as "dense" by Tohru, which he claims is why Koshi hasn't figured out he likes him yet.

I imagine that he would have a slightly feminine voice but still sound like a male.