Sentetsu is a top-ranking member of the Reapers who is known for his hot-blooded attitude and short temper. His background with the yakuza is unexplained, but he shares the passionate loathing of them that most of the Reapers' leadership does; perhaps even more than most of them.
After Kiryu and the HLA have stopped half of the Reapers' operatives, Sentetsu tracks down their base of operations to the Kamuro Castle and shows up on their front door, wielding a submachine gun and demanding that Ibuki come out to face him.
Not wanting to let ibuki risk his life, Kiryu steps out to confront Sentetsu and tells him that he's the one who's been attacking the Reapers' hitmen, so he should take him on instead of Ibuki. Sentetsu gladly agrees and the two men fight outside the HLA's headquarters, though Kiryu proves victorious in the end.
Once Sentetsu is beaten, Ibuki steps out to face him and talks him into giving up his grudge and instead joining the HLA to move on with his life.