Aki and Toshi have an interesting friendship, and one major aspect of it is their attraction to each other. While both are pretty easy on the eyes, their respective attractions go deeper than that. Toshi likes Aki for how confident and brave she is, as well as how she tries to motivate him to do his best. Aki likes Toshi because he's kind, considerate, and loves to help/advise whenever he can. However, Toshi is rather shy and doesn't think he deserves someone like Aki, and she thinks he should have a bit more confidence in himself. So on occasion, she'll do something to make him loosen up and be more outgoing, whether it be teasing him or outright making out with him. Isn't friendship grand?
