Both characters always co-exist and depend on each other; Tainted Soul is incapable of firing tears or dealing damage in almost any manner, and likewise, Tainted Forgotten is incapable of moving on their own. Tainted Soul is able to pick up and move around with, or throw, Tainted Forgotten, similar to how one holds and throws Bob's Rotten Head. Otherwise, Tainted Forgotten has a melee bone weapon like The Forgotten does, which he can swing and throw while on the ground.
Tainted Forgotten will swing his bone at the same time Tainted Soul throws him, and this swing will deal 3x the normal damage, and the impact itself will deal 5 damage.
He is also able to be thrown by Tainted Soul in 360 degrees using the analog stick or mouse controls.
Tainted Forgotten is also completely invulnerable, and instead, Tainted Soul acts as his 'hitbox' and will take damage like any character. Both characters share the same pool of Soul Hearts (they cannot have Red Heart Containers), and if the last one is lost, they both die.
Enemies almost always target Tainted Soul and ignore Tainted Forgotten, but the latter's body can be used to block enemy projectiles and movement.
Both share the same pool of pickups, similar to Jacob and Esau. Tainted Forgotten can pick up items and consumables when he's thrown at them, as well as open Chests or unlock doors.
Tainted Forgotten can be thrown into a door to enter the next room (including Curse Rooms, which deal no damage to Tainted Forgotten), and after entering a room, Tainted Soul will always be holding him. With practice, they can very rapidly traverse the floor, spending a fraction of a second in any cleared room.
When placing a Bomb, it will be placed underneath Tainted Forgotten, and the same is true when dropping trinkets or Cards, etc.
Any items picked up will normally behave as if Tainted Forgotten picked them up and don't directly affect Tainted Soul; for example, orbital familiars will orbit Tainted Forgotten. Speed upgrades on the other hand will apply to Tainted Soul, among other examples. Effects that grant invincibility, i.e. Book of Shadows, will apply to Tainted Soul, whether or not there's visual indication of this.
Tainted Soul can open the door to Mausoleum / Gehenna without losing any health while not holding Tainted Forgotten.
If Tainted Soul picks up Tainted Forgotten while the latter is charging a bone toss, it will throw it once the player releases the attack button.
If Tainted Forgotten's range reaches high enough numbers, his melee bone swing will occur away from him, which can result in an inability to deal damage to enemies too close to him.
Area of effect damage such as Monstrance and 120 Volt apply their effect to Tainted Forgotten, making them incredibly powerful for large crowds and stationary bosses.
Care should be taken when fighting enemies spawned in Devil Rooms, Shops and Black Markets, as Tainted Forgotten will buy anything he's thrown into if possible, with potentially fatal results.
Some pills don't work or are irrelevant because they apply their effects to Tainted Forgotten. Paralysis is one example.
Be careful of Mom's Hands, because they can grab Tainted Forgotten. This will deal no damage, but will still cause teleportation back to the starting room.
Tainted Soul can activate the stop button in Greed Mode without taking damage, at the price of one less coin spawning in the next wave's reward. Tainted Forgotten's body can also be used to activate the button, which does not cost a coin.
Tainted Soul can pass through the mirror in Downpour / Dross II without touching the White Fire Place (in fact, nothing happens when he flies over it), and will keep his life bar, which makes the trek considerably safer.
If Tainted Forgotten is thrown into the White Fire Place, he transforms into The Lost (though he is still invulnerable); Tainted Soul will have Holy Mantle protection (if unlocked), but will die if damage is taken after it's broken like normal.
Items that inflict Fear like Mom's Perfume and 2Spooky should be avoided, because enemies will flee from Tainted Forgotten instead of Tainted Soul, making it harder to hit enemies while also offering Tainted Soul little protection.
Charmed enemies may pursue Tainted Forgotten instead of other enemies, which can be ideal for bringing and keeping them within range of his melee attack.
All contact damage attacks such as Aries and The Virus are tripled in strength and will apply when throwing Tainted Forgotten. The damage is added on top of the normal 5 points his body impact does. Tainted Soul however does not benefit from them at all.
Notable Item Interactions
Birthright: Gives the pair the active item Recall, which will pull Tainted Forgotten into Tainted Soul's hands regardless of distance.
Brimstone: Replaces the bone throw with a short-range brimstone orb.
Chocolate Milk: Instead of throwing Tainted Forgotten instantly, the throw can be charged, which powers up Tainted Forgotten's bone club before throwing them.
Dr. Fetus: Charging an attack will spawn a bomb underneath Tainted Forgotten, which he can fling by swinging again.
Epic Fetus: After being thrown or while sitting around, holding a fire button will prompt Tainted Forgotten to hold a missile and draw a crosshair that travels in the direction being 'fired' in (similar to how Marked works). When the desired distance is reached, let go of the fire button and Tainted Forgotten will chuck the missile in an arc to that location. The missile will not be thrown until the fire button is released. This has no effect on the melee attack.
How to Jump: Will drastically increase Tainted Forgotten's launch distance when used while being thrown.
Ipecac: Tainted Forgotten's bone club will cause explosion when hitting enemies. When he is thrown the explosion may also cause him to ricochet backwards, making retrieval much easier.
Jupiter: Acts solely as a speed boost with no fart effects.
Kamikaze!: Tainted Forgotten is the one who explodes, not Tainted Soul, allowing unlimited use with no downsides as long as the latter keeps his distance.
Kidney Stone: Similarly to The Forgotten, passing a kidney stone will fire a projectile and cause Tainted Forgotten to swing the bone club rapidly for the effect's duration.
King Baby: Follows Tainted Forgotten instead of floating around the room. Shooting familiars still fire towards enemies automatically.
Lead Pencil: Will fire the Lead pencil shot on the 15th swing of the bone club. Holding down the fire button will cause Tainted Forgotten to both swing and charge the bone club. Swinging, then charging and firing, counts as two swings.
Lemegeton Despite the Wisps orbiting the invulnerable Tainted Forgotten, they still take damage unless the Tainted Soul is invulnerable.
Tooth and Nail: Contact damage applies only to Tainted Forgotten, but invincibility is applied to Tainted Soul.
The Ludovico Technique: Spawns a floating tear that slowly and loosely floats towards Tainted Forgotten. If struck with a melee attack, it will grow in size and be launched outwards, dealing damage if it strikes something, before returning again.
Mars: Tainted Forgotten will dash when Tainted Soul triggers it; can be used to control him from a distance.
Mercurius: Allows Tainted Soul to enter a room and immediately throw Tainted Forgotten into the next door, rapidly skipping most of the floor.
Mr. ME!: Mr. ME! will not follow the crosshair shown, and will instead follow Tainted Forgotten. This makes it much harder to direct where Mr. ME! is supposed to go.
My Reflection: All projectiles that would be affected by this item will return to where Tainted Forgotten is.
Panic Button: Active item is used whenever Tainted Forgotten touches an enemy, regardless of damage taken.
Pluto (or any size down effect): Makes Tainted Forgotten smaller, thus making it harder to hit anything by throwing him, while leaving Tainted Soul the same size.
Rocket in a Jar: The bomb will be a rocket only if placed while Tainted Forgotten is being thrown in the air, and will follow the same direction.
Samson's Chains: The chain will attach itself to Tainted Forgotten, allowing Tainted Soul to hide in rocks after throwing him. This also allows safe destruction of potentially damaging objects such as Mushrooms and Polyps.
Serpent's Kiss: The poison touch will affect Tainted Forgotten and will have a chance to drop black hearts upon the foe's death.
Stitches: Will only teleport Tainted Forgotten, which can be used to more accurately reposition Tainted Forgotten without putting Tainted Soul in a difficult position.
Strange Attractor: Will cause enemies to be pulled towards Tainted Forgotten. This is very useful, since it makes dealing damage to enemies easier and protects Tainted Soul, pulling them away and blocking more of their shots.
Tech X: If thrown, Tainted Forgotten's club is surrounded by a ring-shaped laser.
Uranus: Enemies that die due to Tainted Forgotten being thrown at them will also be frozen, and their frozen bodies will usually be knocked into other enemies, causing an ice cascade.
Ventricle Razor: Portals created can be used both while Tainted Soul is holding Tainted Forgotten, which will warp them both, and when Tainted Forgotten is thrown in the portal by himself, which will not warp Tainted Soul; can be very effective for repositioning, especially during boss fights.
Venus: Perpetually charms enemies close to Tainted Forgotten, which synergizes well with the fact that charmed enemies can home in on him, keeping them charmed and in melee range (and away from Tainted Soul) until killed or distracted by another target.
White Pony: Can be used only when Tainted Soul is holding Tainted Forgotten or when Tainted Forgotten was just thrown. Doing so will make a trail of light beams behind Tainted Forgotten as he moves instead of executing the usual charge attack. This makes it possible to deal much more damage with one charge, as Tainted Soul can move the Tainted Forgotten on top of an enemy to make every light beam land on them.
Using the console command "restart 40" starts a new run as only Tainted Soul; he's still unable to fire and otherwise acts identically to The Lost, dying in one hit. It's not normally possible to play the game this way, and the game will crash if Tainted Soul dies while not linked to Tainted Forgotten.
Tainted Soul bears resemblance to a scrapped character from Antibirth named Edith. The mechanic used by Tainted Forgotten also resembles one planned mechanic Edith was going to have.
It was confirmed by both Kilburn and Edmund McMillen that Edith ultimately evolved into Tainted Forgotten during development.
Certain transformations such as Guppy cause Tainted Forgotten to have the transformation's body sprite, making him stand up-right.
The achievement for unlocking Tainted Forgotten is "The Fettered", a possible title for the character.
"Fettered" means "bound" or "shackled", which may be linked to how Tainted Forgotten cannot move on his own and only through Tainted Soul.
Both the Tainted Soul and Tainted Forgotten as a unit are called The Fettered, while Tainted Forgotten is the Dead Weight.
The character's main mechanic of picking up and carrying around a pile of bones could be a reference to Exodus 13:19, in which Moses carries the bones of Joseph with him on his journey with the other Israelites.
Defeating Ultra Greedier as Tainted Forgotten unlocks XIII - Death?, a reversed Major Arcana card associated with longing for an end but not getting it and having to persist without that satisfaction. This could be seen as Isaac not getting an end to his story and having to exist while carrying his bones with him.
Tainted Forgotten's transition screen portrait.
Tainted Forgotten's locked character icon, depicting a closet with some bones sitting outside of it, with the Tainted Soul peaking out of the door
If Gnawed Leaf is active, Tainted Forgotten will transform as expected, but Tainted Soul will also be totally invulnerable while being able to move around freely, even when carrying Tainted Forgotten. This in combination with contact dealing orbitals like Cube of Meat, or items that trigger damage through movement only such as Purgatory, anything can be beaten trivially (if not quickly).
When fighting against Mom's Heart / It Lives, if Tainted Forgotten is not picked up before the appearance of the entrances to Sheol and Cathedral, Tainted Forgotten will automatically enter whichever entrance he rests on, potentially in the direction he doesn't have the corresponding item for.