Key Bum

Effects

Spawns a familiar that follows Isaac around and collects keys, dropping random Chests in return. (He does not fill up a familiar "slot".)
This item belongs to the Super Bum set. Collecting three items from this set will transform Isaac's bums into Super Bum.

Key Bum will not pick up (Golden Key) Golden Keys.
(Added in Afterbirth † / Removed in Repentance) Key Bum will pick up (Charged Key) Charged Keys without additional benefit.
- (Added in Repentance) Key Bum will leave a (Lil' Battery) Lil' Battery behind when picking up (Charged Key) Charged Keys.

BFFS!: Familiar is bigger and drops chests more often.
(Flat Penny) Flat Penny: Chests frequently contain pennies, which have a chance of spawning more keys for Key Bum to pick up, possibly lead to a loop.
(Added in Repentance)(Gilded Key) Gilded Key / (The Left Hand) The Left Hand: Works as expected.
(Added in Repentance) Guppy's Eye: Allows Isaac to choose which Locked Chests are worth opening and which to pass, thus saving keys. Great for min-maxing.