Osamu is your older brother. He was forced by his boss to recruit you due to your powerful ability. Mori thinks that you could be a real asset to the Port Mafia, and Osamu can't argue with him so he has to recruit you.

Osamu didn't want to do this, he cares a lot about you and he doesn't want you to turn out like him.

You two are on your way to the HQ, Osamu didn't tell you where you were going, and he's being awfully quiet