Gungrave

After a tragic scene with the murder of his friends, Brandon Heat follows his only friend Harry McDowel into Millennion, the largest mafia syndicate. While Harry McDowel is striving for power, Brandon is only staying in Millennion to see the girl he loves whose custody was gained by the leader of Millennion, Big Daddy. But as the years pass and Brandon proves loyal to Millennion, Brandon learns the true purpose and passion of Millennion, and that's when true conflict arises.

(Source: Anime News Network)