???, also known as the Blue Baby, is a character unlocked by defeating Mom's Heart ten times. He starts with three Soul Hearts, and his health meter is unique in that he cannot gain red heart containers. If an item would grant him a red heart container (including an empty one), it adds a Soul Heart instead.
Enemies who deal full hearts of damage can be very dangerous in the early floors of the game.
The Poop - While not the most useful item, it can allow ??? to cross small gaps by bombing the Poop from the opposite side, much like one would for Rocks. Furthermore, in rooms with non-flying enemies that chase the player (except Spiders), the poop can be used in a corner of the room making ??? untouchable, allowing the player to kill them without taking damage (if the player moves away from the pile of Poop generated, this will no longer work).
Other utilities to the poop includes generating pickup, repelling projectiles once per room and destroying retracted spikes, benefitting ???'s defensive playstyles.
Bone Hearts grant an empty Bone Heart upon pickup, but they cannot be refilled using Red Hearts.
The amount of Soul Hearts it costs to purchase an item in Devil Room deals is equal to its Red Heart container cost.
??? has an innate Lil Larva effect; destroying a poop while playing as him will spawn a Blue Fly.
Abaddon costs three Soul Hearts and grants six Black Hearts, damage up, fear shot, and no negative effects to ???. Abaddon can be bought safely regardless of current health, because its price can be deducted from the acquired Black Hearts retroactively.
Abaddon will give 2 black hearts on pickup, essentially refunding the cost to obtain the item from a Devil Room.
Lazarus' Rags causes ??? to be resurrected as Lazarus Risen when he dies, allowing the gathering of heart containers. Once this happens, however, you are no longer playing as ???, and therefore cannot get any of his achievements/unlocks. The same effect happens with the use of Judas' Shadow.
Items that directly add Red Hearts to the health bar, such as Yum Heart, Yuck Heart or full health pills, are useless. However Eternal Hearts still act as an extra half a heart and become a full soul heart at the end of the floor making Prayer Card viable.
Cursed Skull will automatically teleport ??? out of the room upon taking any damage, as he doesn't have any Red Hearts. This can be used to easily warp out of the Boss Rush room, but is mostly an annoyance.
Works only at half a soul heart.
Glass Cannon will bring ??? down to half a Soul Heart, and not give back any health upon leaving a room due to ???'s lack of Red Hearts. This item essentially takes away most of the player's health in exchange for a single high damage tear.
This effect no longer applies in Repentance, making it a more viable choice. Even so, caution should be used, as taking damage will cause all subsequent damage taken in the room to be two full soul hearts, making this especially risky for ???.
Guppy's Paw, since requiring Red Hearts to convert, would be a useless item to hold (but should still be picked up and swapped back to get credit towards the Guppy transformation). Guppy's Paw may still be used to activate Broken Remote.
???'s unlock requirements in Rebirth are the same in the original Binding of Isaac.
The name "Blue Baby" had not appeared in the game itself until the Afterbirth expansion, with a co-op baby named as such.
???'s common nickname, Blue Baby, is a reference to blue baby syndrome, in which a baby's skin complexion appears blue from a lack of oxygen in its blood due to a congenital heart defect.
Many of Edmund's older flash games from Newgrounds feature ??? and Brother Bobby. ??? was Edmund's mascot from 1996-2002.
In several cutscenes, Isaac is depicted suffocating inside a chest. ??? is first seen inside an identical chest, and ??? is the follow up boss after Isaac. Blue skin tone is typically used to show suffocation, and X eyes imply that the character is either knocked out or deceased.
After death, the bowels of the deceased will loosen, causing them to defecate themselves. This is most likely the reason ??? starts with The Poop.
Unlike many of the other characters, ??? does not appear in the Bible or any religious scripture. He is the first character with this distinction, and the only one to appear in the original Binding of Isaac - the second being The Lost in Rebirth, third being Keeper in Afterbirth and fourth being The Forgotten in Afterbirth+.
The Ankh will resurrect any character as ???, even before he is unlocked.
Although "Blue Baby" is considered an unofficial nickname for ???, the character is referred to by this name in some of the game's code. One example of this is the image file for the character, named character_006_bluebaby.
???'s look originally came from Dead Baby Dressup, one of Edmund McMillen's first Flash games.
???'s starting health is listed as "-1" on the character selection screen.
As a secret character, ??? does not appear on the character select screen until unlocked.