???, 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.
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.
Ankh is effectively a 
1up! for ???.
Athame can be used to produce black hearts upon taking damage.- Balls of Steel pills or
V - The Hierophant grant two soul hearts, both of which are more useful than normal HP up items.
The 
XI - Strength card gives ??? a permanent soul heart.
Book of Revelations Spawns soul hearts.
Ceremonial Robes costs three Soul Hearts and grants three Black Hearts and damage up. Like Abaddon, this item is always safe to take.
Crown of Light will always activate.
Mitre and 
Daemon's Tail replace useless Red Hearts with Soul and Black Hearts respectively.
Dark Bum converts useless Red Hearts into Soul Hearts and Spiders.
Dead Cat will set Soul Hearts to 1, but can be used to gain devil deals by sacrificing the extra lives.
Dim Bulb is able to be activated every room due to the single charge on 
The Poop, providing a consistent stat boost.
Gimpy has a chance to spawn Soul Hearts when hit.
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.
Maw of the Void can produce Black Hearts.
Midas' Touch makes The Poop create Golden Poop.
Monkey Paw will drop 3 Black Hearts, but must take damage for each one to spawn.
Petrified Poop is useful early on to get more pickups from The Poop every room.
Pyromaniac makes ??? immune to all explosions.
The Polaroid and 
The Negative always activate when taking damage.
The Relic, which is generally useful, is especially useful to ??? as it spawns Soul Hearts.
Satanic Bible generates Black Hearts.
Serpent's Kiss can produce Black Hearts by poisoning enemies.
Squeezy and 
Mr. Dolly increase ???'s tears and can grant Soul Hearts.
Whore of Babylon is always active.
- Items that directly add Red Hearts to the health bar, such as
Yum Heart or full health pills, are useless.
- Blood Donation Machines and Devil Beggars are riskier, as they deplete health that is harder to replenish.
- If ??? has only one heart,
Experimental Treatment has a chance to be lethal.
Cursed Skull will automatically teleport ??? out of the room upon taking any damage, as he doesn't have any Red Hearts.
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.
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.
Judas' Tongue doesn't lower the price of Soul Heart Devil deals.
Little C.H.A.D. is useless unless paired with Dark Bum, where it becomes very useful.
- Sleeping on a bed will give ??? three Soul Hearts.
- 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).
- Bone Hearts grant an empty Bone Heart upon pickup, but they cannot be refilled using Red Hearts.
- ??? appears in the original Binding of Isaac.
- ???'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.
- 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.
Defeat Mom's Heart 10 times
Mom's Heart Unlocks 
The Binding of Isaac: Rebirth 
Achievements
Attributes
Bosses
Cards and Runes
Challenges
Chapters
Characters
Co-op
Items
Item pools
Monsters
Objects
Pickups
Pills
Rooms
Seeds
Transformations
Trinkets
