A hairball, thrown up by Guppy.
Loosely follows behind Isaac. Can be swung like a flail by rapidly spinning Isaac in a circle.
Deals damage when it collides with enemies. If an enemy is killed by the ball, it increases in size up to a maximum limit. The larger the ball becomes, the easier it is to swing, the larger its collision circle becomes, and the higher its damage becomes. Size is reduced at the start of each floor.
Guppys Hairball can push bombs around, so if used with the Remote Detonator, Guppy's Hairball can fling bombs at enemies and the player can blow the bombs up at their will.
- The hairball can be swung while charging slow-firing Fetus or Brimstone shots for added defensibility.
- The hairball goes through walls and enemies, and so can become a substitute for piercing and spectral shots.
- Guppy is Isaac's cat to whom he leaves all his belongings upon death. (as per the screen that pops up if the player dies)