Complete one of the main characters' past to get a very rare random chance to find a Bullet Kin with a red cape replacing a regular Bullet Kin. They will simply wander around and will not shoot at you or deal contact damage either. If the caped Bullet Kin is the last enemy in the room with you, it will eventually disappear. Allow this to happen five times over any number of playthroughs to unlock the Bullet as a playable character.