The Lost

The Lost is one of the 11 Playable Characters in The Binding of Isaac: Rebirth

Unlock Method
The Lost is unlocked via very specific deaths which must be done in order without closing the game or dying erroneously between runs.
 * 1) Isaac must die from a Mulliboom (Muliiboom spawns in the door under you in this seed JTMD EZPF)
 * 2) Magdalene must die from bombing herself in the Caves/Catacombs (can be XVI The Tower tarot card)
 * 3) Judas must die by Mom's foot or Mom's hand during the boss fight. Being killed by anything else in the fight will cause the run to fail and you will need to start over from the beginning.
 * 4) Azazel must kill the fallen then die from Satan's second or third stage. Note that resurrecting with the Ankh, Lazarus' Rags or Judas' Shadow and dying in the appropriate fashion still fails the requirements and you will have to start over.

Info
The Lost appears to be a ghost of Isaac. While playing as the Lost, the player notably begins with flight and without any health, and as such will die as a result of any damage taken. The Lost cannot gain health through any known means, even via soul hearts, black hearts, and HP upgrades. As a side-effect of having no health meter, all deals with the devil can be taken free of charge, despite showing a cost of 3 soul hearts.