The Lost

The Lost is a secret character.

The Lost starts with no health and cannot gain health by any means. Therefore, it will die from any damage taken.

The Lost starts with flight.

3DS The Lost also starts with D4 (if unlocked).

The Lost starts with flight, spectral tears, D4 (If unlocked), and the effect of Holy Mantle (after donating 879 coins to the Greed Donation Machine)
The Lost starts with flight, spectral tears, Eternal D6, and the effect of Holy Mantle (after donating 879 coins to the Greed Donation Machine). This form of Holy Mantle is inherent to the character rather than being a starting item, and as such will be present when switching to The Lost in the middle of the run through the use of Clicker, will not be present when switching away, and cannot be removed by any means.

The Lost can activate the button in Greed mode without taking damage, at the price of one less coin spawning in the next wave's reward.

The Lost can take devil deals and Black Market items for free.

Devil deals and Black Market items are presented as choices. Taking one will despawn all other deals in the room.
- This also applies to devil deals in the first room of the Dark Room; only one of the devil deals spawned by the chests can be taken, regardless of how many chests have been opened.

Notes

Resurrection items like Dead Cat are extremely valuable to The Lost because they are effectively the only health-ups it can get.
Devil deals, while free, still count as making a deal with the devil and will prevent Angel Rooms from opening. Thus, a Credit Card is still recommended, or getting Chaos to acquire angel room items (except Key Piece 1 and Key Piece 2).
Using the IV Bag or a Blood Donation Machine kills The Lost instantly even with the Holy Mantle.
Bedrooms are much more common as The Lost due to its lack of health. While it cannot take advantage of the beds to heal, the guaranteed Crawl Spaces in clean Bedrooms can give more opportunities for items.
If The Lost's Holy Mantle effect is lost while within a Curse Room, closing the game, starting it up, and then continuing the run will restore the Holy Mantle's charge, allowing The Lost to leave the room unharmed.
- The Lost can no longer restore his Holy Mantle by closing the game, starting it up, and then continuing the run.
Using the D4 at all is not recommended, as it will remove the Holy Mantle, making The Lost die in one hit.
After reaching Womb, The Lost becomes slightly easier to play, because the invincibility frames from Holy Mantle scale with the damage that would have been taken.
- Holy Mantle breaking only grants one second of invincibility frames, regardless of the damage source.
Isaac will be replaced by The Lost upon starting a third victory lap; however, Victory Laps do not allow the unlocking of achievements.
- Since the Lost's Holy Mantle isn't intrinsic, the Victory Lap Lost will always die from one hit unless the player has explicitly found the Holy Mantle.
The Lost starts with Eternal D6, even before it's unlocked.
The Lost's Holy Mantle effect is intrinsic to the character once unlocked, and it can't be rerolled.
- Picking up another one does not double the effect.
- Being innate, this Holy Mantle effect is not shown in the pause menu.
Though Devil deals and Black Market items are normally choices, there are four ways to fully or partially bypass this and collect multiple items anyway:
- Making the items free with Credit Card, Coupon, or Mr. ME!.
- Cloning the items with Diplopia or Crooked Penny (all of the cloned items can be taken, but the original items will still be presented as a choice).
- Making the items buyable with pennies via Keeper's Bargain, Strawman, or A Pound of Flesh.
- Your Soul can be used to take an item.

General Strategies

Main article: The Lost (Strategy)

One of the biggest issues of having no health is that many items become completely useless: items that only give health ups, only become effective upon taking damage, or cost health to use provide no benefit to The Lost.
- This is alleviated by the Eternal D6 that The Lost starts with. Eternal D6 has a chance to destroy an item instead of rerolling it, which isn't bad if it's used on a useless item. In fact, on its own, it can only yield a net positive (such as by rerolling a useless item into something useful) or neutral result, such as generating another useless item or destroying it.
Trinkets and Items like Brain Worm that can home-in and target enemies even at a distance are invaluable early in the game, as you do not need to place yourself directly in the enemy's attack path and The Lost's spectral tears pass through most obstacles.
The Lost's ability to fly and have spectral tears makes it extremely useful to take advantage of obstacles.
Anything that grants an extra life (e.g. Dead Cat, Guppy's Collar) is highly useful, allowing The Lost to take hits or safely use certain items that have a chance to kill him, such as Damocles.
Due to The Lost's ability to take 1 Devil Item for free, Devil Rooms and Angel Rooms become equally valuable, unless one is going for Mega Satan.

Item Interactions

Alabaster Box: Alabaster Box does not gain charges from Black Hearts. Soul Hearts still function as expected.
Birthright: Prevents items with the internal tag nolostbr from spawning. In practice, this automatically rerolls most items that aren't beneficial to The Lost, namely ones that only give health, flight, and/or spectral tears or activate upon taking damage.
Brittle Bones: Picking this item up will give a one-time +0.5 tears up ( +0.4 fire rate). This bonus is retained even if the item is later lost or rerolled.
Habit: Holy Mantle will not activate the Habit's effect, making it almost completely useless since the only way that can get its effect to activate is by receiving damage, which would effectively kill the Lost or consume any available extra lives. The same is true for any items that require taking damage.
Heartbreak: The Lost has full benefit of the item. This lets him survive 4 extra hits, each hit adding 1 damage, and the 5th is fatal.
Whore of Babylon: As the Lost has no red hearts (he has half a soul heart), Whore of Babylon is constantly in effect, effectively changing into a damage and speed upgrade.

Unlocking The Lost in Afterbirth

The Lost is unlocked by dying in a Sacrifice Room while holding Missing Poster.

In order to unlock The Lost, you need to unlock Missing Poster.
Missing Poster is unlocked by defeating The Lamb as Isaac.

Looking for Mom's Box in shops makes finding the poster easier.
Glyph of Balance: If all of its conditions are fulfilled, it will change champion drops and room clear rewards to trinkets as long as you don't have one, greatly increasing the chances of finding the Missing Poster.
Glitter Bombs, in conjunction with a large enough or infinite source of bombs (such as a Golden Bomb), have a small chance to drop trinkets, making it easier to find the poster.
- This method got nerfed: Glitter Bombs now have a cap on drops per floor.
It is possible to unlock The Lost while holding Mysterious Paper, although the probability is small since it only has a 1/4 chance to imitate the Missing Poster. Dead Cat and other resurrection items could be useful for extra attempts.
If Isaac happens to teleport away from the sacrifice room right as he dies, then he won't unlock The Lost.
- Sacrificing health for the 12th time in a row and onwards has a 50% chance to teleport you to the Dark Room. Make sure Isaac has no more than 11 hearts before attempting to die in the sacrifice room.
- Sacrificing health for the 6th time in a row will have a 33% to teleport Isaac to the Angel Room. If Isaac loses his last sacrifice and gets teleported to the Angel Room, he will not unlock The Lost.
- Dying to spikes specifically is not required to unlock The Lost, so it is advised to use other sources of damage such as Bombs in these cases.

Unlocking The Lost in Rebirth

The Lost is unlocked by dying 4 times in a row, each time with a certain character dying in a certain circumstance. Deaths must be completed consecutively in the order shown below. Seeds can be used on all but the final step.

Isaac must die to a Mulliboom in Chapter 1.
Magdalene must die to her own Bomb in Chapter 2.
Judas must die to Mom. Being killed by any monster spawned by Mom will spoil the entire attempt at unlocking The Lost.
Finally, Azazel must die to Satan. Being killed by any of the other enemies spawned by Satan or The Fallen will spoil the entire attempt at unlocking The Lost. Using The Bible on Satan does not count as death from Satan. Furthermore, you cannot be killed by Satan's stomp in the third phase. You must die in his second phase.

The Lost will be unlocked after completing the steps shown above.

Notes on unlocking The Lost

Multiple lives (for example, obtained by Dead Cat) must be depleted before the intended death, as deaths while having extra lives do not count as a game loss.
It is highly suggested you do not take items or trinkets with a chance of reviving you after dying, such as Guppy's Collar or Broken Ankh, as these can spoil your attempt if unprepared.
Having any mods installed that trigger the mod flag will make The Lost unobtainable. To fix this problem, all mods have to be removed.
The game cannot be won as any character required to unlock it when unlocking it.
Champion versions of monsters and bosses will count for the death criteria.
Runs can always be safely reset.
Mom's Foot's stomp counts as explosive damage, which makes Pyromaniac a problematic choice when trying to complete Step 3.
Picking the wrong character while unlocking him doesn't mess up the unlocking process.^[1]
Quitting and Restarting the game in the middle of the unlocking process doesn't mess it up.^[1]
Picking up Lazarus' Rags, Judas' Shadow, or Ankh should be avoided. These items will revive the current character as a different one and therefore prevent the current unlock-step from being fulfilled.
Exploit: If you die during a required run, but hit the "start" button just as you die and reset before it shows Isaac's Last Will, it will not count as a death that spoils the run; instead, it counts as a restart.

File Manipulation

By changing a file parameter, an unsuccessful try can be undone:

Backup the file Options.ini found in the Documents\My Games\Binding of Isaac Rebirth by making a copy of it in the same folder.

Open the original file with Notepad and change the SteamCloud parameter in it to 0. This will disable Steam Cloud gamesaves synchronization.

After an unsuccessful try, delete the original file and rename the backup copy with the name of the original file: Options.ini
Repeat steps 1 after each successful run until you unlock The Lost.
When finished, open the original file and restore the SteamCloud parameter in it to 1. This will enable Steam Cloud gamesaves synchronization again.

Helpful Seeds For Rebirth

Isaac

PC GPE3 2T1H (2 Mullibooms south of starting point. Use the sacrifice room above to damage yourself first)

PC PS4 JTMD EZPF (One mulliboom directly below the starting. Go left first to get weakened)

PC 4Z93 R21B (1 down 1 right from start contains mullibooms)

PC GH9Y FSA8 (series of rooms containing mullibooms going down)

PC Q2V4 YZET (Cellar needs to be unlocked, 2x2 room full of mullibooms (about 6/7) when going up)

Vita GQHN NMKN (Go left, 4+ Mullibooms)

3DS 6Z2L AZ6J (Go left, 4+ Mullibooms)

3DS VNSM AHHG (Go up, weaken yourself at the spikes, then go up and die from the Mulliboom)

3DS PXRT XH49 (Go up, weaken yourself with the Frowning Gapers, then go down and die to the Mulliboom)

iOS 3KY7 JPBJ (Go right and weaken yourself, then go left and there are 2 mullibooms)

Maggy

PC ALFA 0NTS (Recommended Normal difficulty. Pyro in the first Treasure Room, adjacent to spawn. Go left on basement 2 for we need to go deeper in the secret room.)

PS4 FDRG 0CTE (Pyro in the first Treasure Room, adjacent to spawn)

Vita DXVW MQCF (Pyro in the first Treasure Room)

3DS YGDC T8SL (10 bombs in the first Treasure Room)

iOS E1NZ N3MA (bomb in second floor shop, the fires should give enough pennies to buy the bomb.)

Judas

PC YR8E YXDV (Polyphemus in the first Treasure Room. Do not take devil deals, as Trinity Shield and Sacred Heart will be available in Angel Rooms. Keep the Book of Belial to ensure they show up. Also, from the pills from Mom's Coin Purse (1st boss, Duke of Flies), take the orange (Health Up) then the white one (Full Health), ignore the blue ones (Speed Down)).

PC N0G3 VLD4 (Both modes) - Dr. Fetus on floor 1, and Ipecac on floor 2.

PS4 DMTM NRD9 (Do not make a Devil Room deal in Basement II will spawn an Angel Room with Holy Grail in Caves I. Collecting everything grant huge tears.

3DS 180Y 7RDC (Lots of health early (Rosary in first Treasure room), good pickups (Mulligan is in second Treasure room, Dr. Fetus and Mr. Mega in the third and fourth). Skip the first Devil deal, as the Angel deals are Sacred Heart, Mitre, and Scapular. Remember to trigger scapular before entering the Mom fight if you take it, as the immediate foot stomp may leave you with the bonus soul heart.)

iOS 0FP1 C9JK (The first boss room gives a heart, the first devil deal gives dead cat so you can survive.)

Azazel

Using a seed on this step will not count towards unlocking The Lost.

The Lost

PC HNA6 WEK7 (Dead Cat and Holy Mantle on first floor, helpful for practice runs)

PC 616S GROG (Holy Mantle right of spawn, helpful for practice runs)

Unlockable Achievements

The D100 - Complete the Boss Rush as The Lost.

-0- Baby - Defeat Mom's Heart on Hard mode as The Lost.

The Mind - Defeat Satan as The Lost.

Isaac's Heart - Defeat Isaac as The Lost.

The Soul - Defeat The Lamb as The Lost.

The Body - Defeat ??? as The Lost.

Zodiac - Defeat Ultra Greed as The Lost.

Dad's Lost Coin - Defeat Ultra Greedier as The Lost.

Sworn Protector - Defeat Hush as The Lost.

White Baby - Defeat Mega Satan as The Lost.

Holy Card - Defeat Delirium as The Lost.

Lost Soul - Defeat Mother as The Lost.

Hungry Soul - Defeat The Beast as The Lost.

The Baleful - Use Red Key (etc.) to open the hidden closet in Home as The Lost.

Godhead - Earn all 5 / 9 / 10 / 12 Completion Marks on Hard mode as The Lost.

Unlockable Starting Items

Donate 879 Coins to the Greed Donation Machine - Start with Holy Mantle
Blow up 30 Slot Machine - Start with D4

Gallery

The Lost's transition screen and boss vs. portrait.
The Lost on the character select screen.
The Lost on the boss vs. screen prior to Repentance.
The Lost on the next floor screen prior to Repentance.
Isaac briefly appears as The Lost during the cutscene preceding The Beast.

Trivia

The Lost's name and appearance hints that he is a ghost/soul/specter/spirit-type entity so lost it couldn't even find its way to the afterlife.
- The name could also be linked to the fact that Isaac was presumed missing by his mother (as seen in Ending 14, and the fact The Lost is unlocked using Missing Poster), while in fact he was actually dead (as seen in Ending 17 and Ending 20).
When The Lost's unlock requirements were discovered through the use of data mining, Edmund McMillen and Tyrone Rodriguez both expressed their disappointment, considering it impatient and "like swallowing a hundred dollar steak in one bite".^[2]
Right before the start of a run, some images appear to hint to unlocking The Lost, including the Missing Poster and The Lost itself.
Dying in a Sacrifice Room while holding the Missing Poster would yield a puzzle piece on Isaac's last will. These puzzle pieces, when put together, created the last wills of characters dying in the method used to unlock The Lost.
Before version 1.02, The Lost would die if a run was exited then continued.
When The Lost picks up an item, tarot card, or pill, you can see its arms that do not connect to its body.
Internally, The Lost has half a soul heart, which is hidden behind a permanent Curse of the Unknown-like effect. The singular soul heart is why The Lost can "pick up" soul hearts, black hearts, and eternal hearts. There are no ways to increase this health, though, as The Lost's health is constantly reset back to half a soul heart.
- Devil deals appear as requiring three soul hearts each, as The Lost technically only has half of an invisible soul heart. This is cosmetic only, as devil deals are still free.
- The Lost can pick up soul hearts within the small timeframe of invincibility granted by Astral Projection after getting hit.
- The Lost can also do this while defying death using Berserk!
- The Lost's health was visible on the HUD in V1.7.5 but was made invisible again in V1.7.6.
A loading screen (when Isaac dreams) depicts the "Voice of God" telling Isaac's Mom to kill her son as The Lost speaking to her from the attic.
The Lost's starting health is listed as "?" on the character selection screen.
As a secret character, The Lost does not appear on the character select screen until unlocked.
- He shares this trait with ???, Keeper, and The Forgotten.