Ah, Tsuyoshi-kun… it's somehow frightening to think what he would be like as a teenager…
Tsuyoshi has been Hayama's friend since early childhood, and he's also good friends with Sana. He has always stood by Hayama and tried his best to help him through his family problems in spite of Hayama's sort of cold attitude towards him (at least early on).
Tsuyoshi has well-hidden family problems that only come to the surface when his parents get divorced. It is then we learn how strong he can be for the sake of others, and that his usual cheerfulness is not the result of an easy life. In spite of everything that's wrong, he tries to be a good son and a good "father figure" for his younger sister, Aono-chan. He's even forgiving of his dad, even though most kids in his position would be a lot colder towards him, at least on the outside.
I admit that sometimes Tsuyoshi sort of gets on my nerves, but I think that's only when he's supposed to be annoying in his own dorky kind of way. I generally like him a lot because he's such a good person and the type of friend Sana and Hayama need. He looks out for both of them and their relationship, and he's generally very thoughtful of how they feel.
Although it's hard to tell by just looking at him, underneath Tsuyoshi-kun's kind personality (and cute, slightly dorky appearance) lies a time bomb waiting to smash you on the head with the nearest school desk. If something upsets him greatly, like hearing someone insulting his mom, he gets a crazy glint in his eyes (or on his glasses, actually) and goes berserk ("Boku ga okocchau yooo!"). At that point it seems he usually manages to have a desk handy, and only a well-placed chop on the head from Hayama will calm him down. He has trouble controlling his behavior, but all's well since Hayama assures him he'll always be there to chop him on the head. ^_^
