The Sole Survivor has to confront him as part of the quest The Silver Shroud as a final boss. After the Sole Survivor decides to help Kent Connolly in his mission to bring law in the streets of Goodneighbor and kills some minor agents of his being Wayne Delancy and AJ, Sinjin uses Northy and Smiling Kate to do his dirty work.
Unfortunately for him, the Sole Survivor kills both of them and is able to retrieve clues about him. Vengeful, he personally captures Kent in Goodneighbor and takes him to his hideout, the Milton General hospital. He holds him captive until he is killed by the Sole Survivor.
Interactions with the player characterEdit
There are a few ways to save Kent. One is to speak as the Shroud to scare away Avery and Sinjin's lackeys, then quickly kill Sinjin before he can kill Kent, to convince him to let him free and blame all on the Sole Survivor with a high Charisma, or convince him to attempt to kill the player character first, and use jet to injure him and Avery. Alternatively, one can quickly kill Kent using V.A.T.S. Doing so will confuse and terrify Sinjin as well as Avery and the lackeys.
Sinjin appears only in Fallout 4.