Super Bum

Completing this transformation will take the three bums and replace them with the Super Bum familiar, who automatically collects all coins, red hearts, and keys in the room and drops twice as many rewards as the regular bums would have produced.

Trivia

 * This is the only transformation that does not affect Isaac himself, but instead transforms (and even fuses) certain familiars.
 * This is the only transformation that has a unique animation.