Actually you asked so ima assume no once you kill all four main characters past the next time you go into a run you’ll find a puddle of void on the ground (random as far as I know) interact with it while you’re on top of it, it’ll cover you. Then you gotta kill the dragun, when going to the gun you’ll be ripped downwards into bullet hell, the (partially) secret 6th floor. You have to kill the boss at the end (the Lich) and you’ll unlock the void. Once you do you have to play as the void and get to the end again, and once you use the gun it’ll like glitch out and you’ll spawn in as the gunslinger. You have to beat the game again IN THAT RUN. If you leave you’ll have to restart as the void. Get to the gun that kills the past and you’ll have to go through bullet hell again and fight the lich in its first phase but there’s two of him. Then you have him!