Tsuyoshi is Akito's best friend. The two are an odd combination because Tsuyoshi is very cheerful, honest and nice, which is exactly the opposite of how everyone perceives Akito, and Tsuyoshi hardly ever participates in the antics of Akito's henchmen. However, much like Akito, Tsuyoshi has a tough family situation, as his parents are divorcing. Instead of dealing with his problems like Akito, he becomes kinder for the sake of others.

Because of his ability to be strong and kind in bad situations, Tsuyoshi can sense that there is good within Akito and tries to convince others of what he sees, and tries to convince Sana that Akito isn't a bad guy, he's just misunderstood. Don't mistake Tsuyoshi for all happiness and sunshine though, for if one makes a disparaging remark about his family, especially, his mother, he produces an explosive temper that cannot be quelled.

Later in the story his name becomes Tsuyoshi Sasaki, and in kanji, 佐々木剛]
(Source: kodocha.tv)