This character is perhaps the hardest to unlock.
How to Unlock The Gunslinger
Unlock Hint 1: You need to have unlocked a previous secret character AND a secret floor, which requires, at the very least, defeating the Dragun.
Unlock Hint 2: Unlocking the character involves messing with the laws of the universe while already in an exceptionally arcane place.
Unlock Hint 3: You need to play as the Paradox and go to the secret 6th Chamber.
Unlock Solution: Defeat the final boss in Bullet Hell as the Paradox to unlock this character.