Tsuyoshi is Akito's best friend. The two make an unusual pair because Tsuyoshi is very cheerful, honest, and kind, which contrasts sharply with how everyone views Akito. Tsuyoshi rarely joins in the mischief of Akito's group. However, similar to Akito, Tsuyoshi faces a difficult family situation as his parents are going through a divorce. Instead of reacting like Akito, Tsuyoshi becomes even kinder to support those around him.
Due to his strength and kindness in tough situations, Tsuyoshi believes there is goodness within Akito and tries to convince others of this. He also attempts to persuade Sana that Akito isn't a bad person, just misunderstood. Despite his generally positive demeanor, Tsuyoshi has a fierce temper if someone insults his family, especially his mother.
Later in the story, his name becomes Tsuyoshi Sasaki, written in kanji as 佐々木剛. (Source: kodocha.tv)
