Fukami was hit by a stray bullet. After he fired at the rival gang, they fired back. So, it could be any of the rival gang members who shot him. Yuusuke ran at him and tackled him to save him, but he had already been shot in the head. Once Fukami regained his memories he simply goes on living with Yuusuke. That is how the manga ends, with Fukami slowly regaining memories, but doing nothing about them and staying with Yuusuke. It would have been impossible for Fukami to have left or been kicked out of the Yakuza and live a "normal" life if he had not been shot and lost his memories. His injury was a blessing and I think, as he regains more and more of his memories, he sees it as one too. He knows his life was shit and he also knows he had no other path at the time, except what the Yakuza offered him. Now, however, he knows he has other choices and other paths. In particular, his path with Yuusuke. I think that is why he is choosing to ignore his memories and just stay with Yuusuke.
thanks!
tank you
though I skipped a lot of yakuza talking I feel sad at the ending but I couldn't understand a lot of things like who shot fukami san and what happened to him after he regained his memories but I can say that I wasnt bored reading this very long manga