I did not understand what happened to Hirokawa in the past???

He was being bullied in his previous school and when he jumped on stairs he pulled a guy with him. He was injured and had to got to the hospital. The guy who he pulled with him didn't let Hirokawa to go back to that school. And Hirokawa was transferred to Yamato's school.

Sometime before falling down the stairs he met Yamato in the rain

aaah right! thats true i forgot that :D