After at least one characters past has been killed, refer to how to construct the bullet here
a new type of enemy bullet kin will begin spawning in the Gungeon, the red caped bullet kin. They have a distinctive red cape and a pistol which they will not fire.
They are hard to discern from other enemy bullets so keep a sharp eye, as your task is to
NOT kill them. You must clear out the rest of the room, and they will disappear. Do this 5 times and you will have unlocked the Bullet as a playable character.