He has a total of five items related to him in the original game (6 in Rebirth) and upon collecting three (excluding Guppy's Hair Ball), will transform Isaac into Guppy.
Transforming into Guppy
Dead Cat, Guppy's Tail, Guppy's Head or Guppy's Paw have a unique feature: Isaac can permanently transform himself into Guppy when he collects any three of them in a single playthrough. All of them can potentially be found in the Devil Room, Curse Room and Red Chests. All of the Guppy items except Dead Cat are exclusive to Wrath of the Lamb.
Transforming into Guppy unlocks Guppy's Hair Ball.
Tips for Transforming
- Having picked the items up at some point is the only thing that matters, so even though Guppy's Head and Paw are both activated items, Isaac will still transform even if he picked up one of them earlier and left it behind, or rerolled it with The Dice.
- Therefore, the latter can be a method to acquire another Guppy item. For instance, if Isaac has The Dice and found Guppy's Paw in the Devil Room, he can pick it up, then switch back to the Dice to reroll it for another Devil Room item, potentially being another Guppy item.
- The Left Hand Trinket can increase the likelihood of becoming Guppy, since it turns all future Chests into Red Chests, which can contain Guppy items. This has good synergy with Guppy's Tail as well, since it increases the number of Chests appearing, which become Red Chests with the Left Hand.
- If Isaac gets a duplicate of Guppy's Tail it will be counted toward becoming Guppy a second time, unlike any of the other Guppy items.
- The Parasite completely eliminates the ability to spawn Attack Flies when Isaac's tears strike an enemy. Tears from Familiars will still produce Attack Flies.
- Chocolate Milk makes uncharged Attack Flies deal no damage. If charged, they will deal the bonus damage associated with charging shots. Uncharged Attack Flies still instant kill Black Fly, Attack Fly, Pooter and Fat Fly.
- Shooters such as Brother Bobby, Demon Baby, Gish, and Little Steve will spawn attack flies when their tears hit.
- Fly spawning effect stacks with The Mulligan item.
- You can't spawn two flies with one shot.
- Technology/Technology 2 - The high fire rate of the lasers greatly increase the amount of blue flies spawned, especially for Technology 2.
Found in the Devil Room, Curse Room, Red Chests, and as a drop from Super Pride. Isaac's heart containers are set to 1 and he is given 9 lives, as well as a dead cat head Familiar. Health upgrades can be taken as normal, but upon dying, his heart containers will be reset to 1 again.
Found in the Devil Room, Curse Room or Red Chests. Once collected, Chests have a 1/3 chance of replacing any other non-specific Pick Up.
Found in the Devil Room, Curse Room or Red Chests. Spawns 2-4 Attack Flies upon activation to damage enemies. Recharges after clearing one room.
Loosely follows behind Isaac. Can be swung like a flail by Isaac rapidly spinning 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. Collecting Guppy's Hairball does not count towards becoming Guppy in the Flash version, but it does in Rebirth.
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Guppy's Collar has a 50% chance to revive the player with 1/2 hearts. They retain their max HP. The collar, unlike the 1UP or Dead Cat, is not used in this revival, meaning that if luck swings the player's way, this item can activate many times.
- If the third Guppy item Isaac picks up is an activated item, the game may bug, so he may have to switch out the final active item before becoming Guppy.
- Since the game's Prologue doesn't tell anything about Guppy, this cat is probably long dead by the time Isaac explores the world under the trapdoor, since pieces of him can be found throughout the adventure and eventually Isaac can also find Dead Cat which represents Guppy himself. Given that in Isaac's death note the boy leave everything to Guppy, it's also possible that Guppy is Isaac's imaginary friend .
- Guppy is actually a reference to Edmund McMillen's own pet cat called Guppy. Guppy passed away on August 19, 2014 at the age of 12.
- Guppy appears in the Castle In The Darkness Prologue, where his curse brings you to an alternate level.
"Guppy's Hairball" - Collect three Guppy-related items in a single playthrough.