Both he and Kashuu Kiyomitsu are personifications of swords owned by Okita Souji. Unlike Kiyomitsu, who resents Okita for abandoning him, Yasusada is very fond of Okita and hasn't gotten over his former master's death, to the point of dressing and occasionally acting like him.
Normally, he has a carefree personality with his comrades, but is ruthless and sadistic in battles. Of course, artists tend to portray him as someone who's completely obsessed with Okita (to the point of being dismissive with the Saniwa) and who gets violent for no reason. Either that, or he's presented as a foil to Kiyomitsu.
