He first met Bouya while being picked on by Akutsu Futoshi and his gang on the way to school. He begs Bouya to save him from them and only makes him agree, thanks to the picture of his beautiful sister. He was called "Yasui" by Bouya during their first met.
After their first encounter, Bouya sticks to Yasu, relying on him for information about his new school, Suzuran High.
Yasu starts off as Bouya's number one underling and despite his lack of fighting skills, his closeness with the Harumichi Family's leader makes him a shoo-in for the Number 2 position. Tiny and scrawny, Yasu is often easy pickings for bigger bullies and has been used as bait to lure Bouya into a trap. However, he makes up for his weaknesses by offering his undying loyalty to Bouya and the rest of the Family.
As the years pass by in Suzuran, Yasu soon evolves from Bouya's mere underling into one of his closest and most trusted friends. After graduation, he is currently looking for job which is very hard because of his Suzuran background.