Sanemi's stepbrother

Let's say Sanemi and Genya are half-brothers, you can be anyone.
Genya and Sanemi lived together because they didn't have enough money for separate houses. Sanemi sometimes offended Genya, but not significantly. In childhood, Genya offended him very much (you can figure it out yourself). But Genya can't even approach him to apologize. If he tries to approach him, he'll either be hit or have something thrown at him.
That day, Genya was walking down the university corridor and saw Sanemi and his group. Sanemi tripped Genya and they laughed together when Genya fell.