Dweller
Dwellers are enemies who resemble 
Isaac, and behave like the player would; a Dweller will move along the cardinal directions in order to line up with Isaac before shooting tears towards him, and may start with one passive item in a select list which can alter their behavior.
The list of items Dwellers can use and how they are affected are as follows:
The Inner Eye: Makes the Dweller shoot 3 tears at once, at a slower rate.
Spoon Bender: Gives the Dweller's tears homing.
Number One: Gives the Dweller yellow tears with a very short range.
Brother Bobby: Spawns a Dweller Brother to accompany the Dweller.
Technology: Replaces the Dweller's tears with a Technology laser. The Dweller will make a telegraph before firing by having their eye flash.
Polyphemus: Makes the Dweller shoot very large tears that deal a whole heart of damage, at a very slow rate.
Cricket's Body: Makes the Dweller shoot low-ranged tears that split into 4 smaller tears upon landing. The tear, prior to splitting, deals a whole heart of damage.
Cursed Eye: Gives the Dweller a charged attack that shoots 4 tears in a line. The Dweller has a chance to teleport away when they take damage while charging; after teleporting they will never appear again.
Soy Milk: Makes the Dweller shoot small tears at a very high rate of fire, even faster than Number One.
Lost Contact: Causes the Dweller's tears to collide with Isaac's tears if they intersect, destroying both projectiles.
Notes
- The type of item a Dweller will use is determined by the room layout.
- The IDs for each Dweller variant is based on the matching ID for each item itself such as 169.80.2 for The Inner Eye.
- In order, the IDs of the Dweller variants are: 2, 3, 6, 8, 68, 169, 224, 316, 330, and 213.
Dweller Brother
Dweller Brother accompany Dwellers when they use Brother Bobby. They will follow Dwellers who spawned them and will fire blood projectiles alongside them. They will automatically die when their Dweller is killed, although they can be killed separately as well.
Variants
Resident
Residents are variants of Dwellers that behave identically but use active items instead. They can use their active multiple times, using them when certain criteria are met and enough time has passed since their last use.
Tammy's Head: Periodically when shooting, the Resident will fire 10 large tears in a ring.
Bob's Rotten Head: Periodically when shooting, the Resident will fire a Bob's Rotten Head shot that explodes and leaves a cloud of toxic gas.
Lemon Mishap: When Isaac is close enough, the Resident will spawn a puddle of yellow creep.
Book of Shadows: When damaged, the Resident will become invulnerable for a period of time.
The Nail: On room start, the Resident will gain 5 maximum health, have their movement speed reduced, and deal a whole heart of damage with their attacks and on contact. The Resident will also be able to walk over 
Rocks, crushing them.
The Pinking Shears: On room start, the Resident will gain flight and spawn a headless body that chases Isaac.
Guppy's Head: Shortly after the room starts, the Resident will spawn 3-5 
Attack Flies.
Crack the Sky: Shortly after the room starts, the Resident will spawn 6 light beams in random positions around the room.
How to Jump: Periodically, the Resident will quickly jump towards Isaac.
Box of Spiders: When Isaac is close enough, the Resident will spawn 2-4 
Spiders.
Red Candle: Periodically when shooting, the Resident will launch an extinguishable flame.
Butter Bean: When Isaac is close enough, the Resident will fart and deal knockback to Isaac and his tears.
Friend Finder: On room start, the Resident will spawn a random Dweller with their own item which mimics the Resident's movements and shoots tears alongside them.
Notes
- The type of item a Resident will use is determined by the room layout.
- Like Dwellers, IDs for each Resident variant is based on the matching ID for each item itself.
- In order, the IDs of the Resident variants are: 56, 58, 83, 107, 145, 160, 282, 288, 289, 294, and 687.
Missing Link
Missing Links are a tainted fusion of Dwellers and 
Fishaacs, found in the 
Caves during the 
Ascent. They behave like the former, lining up with Isaac to shoot towards him, and using higher tier items shortly after spawning.
However, they also feature Fishaac's unique health mechanic: they activate invincibility frames after being hit, can only take 1 HP worth of damage per hit regardless of damage source, and leaving behind a body upon death. They require a total of 5 hits to be killed.
The items they can use and their respective effects are as follows:
Harlequin Baby: Spawns a Harley to accompany the Missing Link.
The Ludovico Technique: Instead of lining up to fire tears at Isaac, the Missing Link will create a Ludovico Technique tear that erratically chases after Isaac, generally moving in cardinal directions, while the Missing Link also tries to stay further away from Isaac. If the Ludovico Technique tear is destroyed, the Missing Link will spawn another one.
Godhead: The Missing Link's tears are now homing and gain an aura that acts like that of 
Dogma's Godhead tears, creating a Crack the Sky beam on Isaac's position if he stays inside it for too long.
Continuum: The Missing Link's tears become spectral, wrap around to the other side of the room after passing through a wall, and have much longer range.
Tech X: Instead of firing tears, the Missing Link charges and fires Tech X rings that scale in size with how long they've charged.
Varicose Veins: When hit, the Missing Link fires a ring of 10 tears around it and leaves a few puddles of damaging red creep on the ground where it stands.
Notes
- The type of item a Missing Link will use is determined by the room layout.
- Like Dwellers, IDs for each Missing Link variant is based on the matching ID for each item itself.
- In order, the IDs of the Missing Link variants are: 167, 329, 331, 369, 395, and 452.
Harley
Harleys accompany Missing Links that have Harlequin Baby. They will follow the Missing Link who spawned them and fire 2 blood projectiles in a V shape alongside them. They will automatically die when their Missing Link is killed, although they can be killed separately as well.
Trivia
- Lost Contact Dwellers were originally unused in the Afterbirth+ release of Fiend Folio.
- Several variants of Residents, Dwellers, and Missing Links with joke items are hidden away in the code. They can be spawned in through entering commands into the debug console.
- The joke items for Dwellers are:
Euthanasia: Makes the Dweller shoot needle tears that instantly kill Isaac.
Plan C: Isaac dies right after the Dweller uses this. The Dweller will then die a short time after.
The Fiend Folio: The Dweller gains a Dweller Brother, a massive tears up and now fire lobbed, homing tears that are shot by volleys of 3 and split into 4 smaller tears upon landing.
- The joke items for Residents are:
IPad: Upon using this, Isaac is instantly sent to the bottom of the room.
A.V.G.M.: The Resident will repeatedly use A.V.G.M., occasionally spawning 
Cursed Pennies while turning the lights on and off.
- Missing Link has one joke item:
R Key: On use, the game warps back to the first floor with only the Missing Link present in the room.
- The joke active items for Dwellers and Missing Links were created before Residents were made.
- The joke items for Dwellers are:
Gallery
Dweller with Inner Eye.
Dweller with Spoon Bender.
Dweller with Number One.
Dweller with Technology.
Dweller with Polyphemus.
Dweller with Cricket's Body.
Dweller with Cursed Eye.
Dweller with Soy Milk.
Dweller with Lost Contact.
Dweller with Euthanasia.
Resident after using The Nail.
Missing Link with Ludovico Technique.
Missing Link with Godhead.
Missing Link with Continuum.
Missing Link with Tech X.
Missing Link with Varicose Veins.
A Missing Link's corpse.
Concept art for Dweller.
Concept art for Resident.
