Gives Isaac a Dumpy familiar, which follows behind him. When Isaac is about to take damage (as with Panic Button), Lil Dumpy farts, knocking back nearby enemies and shots while also propelling itself in a random direction. Enemies that collide with obstacles or walls take 10 + 2 * (floor - 1) damage. Pushed shots that hit an enemy deal 6 damage normally and 7 damage if they would deal a full heart of damage to Isaac.
Lil Dumpy has a larger, hidden secondary radius that may randomly trigger its fart effect if an enemy projectile enters it.
After farting, Lil Dumpy goes on cooldown and "falls asleep" until Isaac walks up to it to wake it up. Lil Dumpy resets upon entering a new room.
Lil Dumpy deals contact damage, which depends on its speed. If it hits an enemy right after farting, it will deal 8.33 damage.
Notes
Does not prevent damage from environmental hazards, including Spiked Chests.
Lil Dumpy's position behind Isaac can overlap with other Familiars or other copies of itself.
Dormant Lil Dumpy can come into contact with and be pushed around by enemies, but doesn't deal contact damage.
Lil Dumpy can get stuck over rocks and pits, becoming unreachable for the rest of the room.
Synergies
BFFS!: The contact damage is boosted greatly. For instance, if the familiar hits an enemy right after farting, it will now deal 33.33 damage.
Gigante Bean: Applies to Lil Dumpy and causes the fart to become much bigger and also deal 10 damage.
The Twins: Can duplicate Lil Dumpy, practically adding an extra charge to Lil Dumpy's Holy Mantle-like protection.
Ventricle Razor / Other methods of teleportation: Lil Dumpy will quickly fly to Isaac instead of teleporting, dealing damage to everything in its path.
Lil Dumpy is from the Antibirth mod. In Antibirth, Lil Dumpy instead wandered around the room randomly and would fart when blocking a projectile, which could poison or confuse nearby enemies as well as knock them back and send Lil Dumpy flying.
The design of this item is also based off of Dumplings, a common enemy present in Antibirth that does not appear in Repentance. Dumplings are a creature featured in multiple of Edmund McMillen's previous Flash games, and even having a game of their own.
Prior to V1.7.8, Lil Dumpy would instead behave like its Antibirth version.
Panic Button), Lil Dumpy farts, knocking back nearby enemies and shots while also propelling itself in a random direction. Enemies that collide with obstacles or walls take 
Spiked Chests.
BFFS!: The contact damage is boosted greatly. For instance, if the familiar hits an enemy right after farting, it will now deal 33.33 damage.
Gigante Bean: Applies to Lil Dumpy and causes the fart to become much bigger and also deal 10 damage.
The Twins: Can duplicate Lil Dumpy, practically adding an extra charge to Lil Dumpy's 
Holy Mantle-like protection.
Box of Friends: Will copy Lil Dumpy for extra protection, this process can be repeated many times to make Isaac near-invincible in Greed mode, Greedier mode, 
Boss Rush or with 
4.5 Volt.
Ventricle Razor / Other methods of teleportation: Lil Dumpy will quickly fly to Isaac instead of teleporting, dealing damage to everything in its path.
The Binding of Isaac: Rebirth
