Not to be confused with Leech (enemy)
A familiar that follows enemies, dealing contact damage. When it kills an enemy, Isaac is restores for half a red heart.
- Works well against Globin, because it can regenerate a bit faster than The Leech can kill it, and the player gets half a heart every time the Leech downs Globin, giving the player an infinite health supply if you just let The Leech kill it continuously and don't finish it off.
- The Leech makes it easy to abuse Arcades, as it is possible to use a Blood Donation Machine to get coins, play the Shell Game, and let the Leech kill the Attack Fly that is spawned by losing the Shell Game or Slot Machine to regain the half-heart used for the Blood Donation Machine, allowing for unlimited usage of the Arcade until either the Blood Donation Machine or the Shell Game Beggar is depleted/gone.