Lost Soul

For the trinket with a similar name, see Found Soul. For another trinket with a similar name, see Your Soul.

Lost Soul is an unlockable passive item added in The Binding of Isaac: Repentance.

Effects

Creates a ghostly familiar that closely follows behind Isaac. It dies in one hit, respawning at the start of the next floor.
If Lost Soul survives until the start of the next floor, it rewards Isaac with one of the following:
- 3 Soul Hearts.
- 2 Eternal Hearts.
- A Treasure Room item or an Angel Room item.

Lost Soul hides behind Isaac's back, so shooting left will cause Lost Soul to be on your right side and vice versa.
- Also, if Isaac is moving right but shooting left, then his hitbox will completely absorb Lost Soul's hitbox, making it significantly harder for enemies to hit it.
Floors completed during the Ascent count.
Waiting until after the floor is cleared before picking up Lost Soul nearly guarantees a reward payout on the next floor.
The ghost can take explosive damage from Isaac, e.g. via placing a Bomb.
Lost Soul's position behind Isaac can overlap with other Familiars.
Leaving the Mother's Shadow chase sequence respawns Lost Soul.
Swapping the item as Tainted Isaac and picking it up again respawns Lost Soul.
Spiked Rocks will kill Lost Soul.

BFFS!: Gives Lost Soul a once-per-room shield, similar to Holy Mantle.
- If BFFS! is dropped, the shield will still remain active until taking one hit of damage, acting similarly to Holy Card.
Forget Me Now/R Key: Restarting a floor/run causes Lost Soul to give a reward if it's alive when used.

Box of Friends: Cannot duplicate the ghost and won't spawn a Demon Baby, even when the ghost is dead.
Bursting Sack / Skatole: Does not provide immunity to contact damage from spiders/flies to Lost Soul.
Chaos: Items dropped by Lost Soul are drawn from a random pool.
Child Leash/Duct Tape/Friendship Necklace/King Baby/RC Remote/The Twins: No effect.
Diplopia / Crooked Penny: Picking up multiple Lost Souls has no effect, even if one died previously.
Missing No.: If a new floor is entered and Missing No. rerolls into this item, it will immediately give a reward.
Tainted Forgotten: The ghost follows Tainted Forgotten, making it very difficult to protect.

Lost Soul originates from the Antibirth mod. In Antibirth, multiple copies of the item would grant multiple familiars.

Bug!	If Lost Soul is killed before the segment with Mother's Shadow, it will respawn after Isaac returns to the normal floor.

Bug!	Swapping Lost Soul for another item as Tainted Isaac and picking it back up again will cause it to respawn if it died previously. This can also be accomplished with Butter!.