Despite starting with only black hearts, Esau Jr. is able to gain heart containers.
Coins, bombs, and keys are shared between the two characters, but everything else (including hearts, items, cards/pills, and trinkets) are not shared and apply only to the character that collects them.
Esau Jr. will have the same base stats, character-specific abilities and passive starting items (such as Lilith's blindfold and Incubus) as the original character.
The only exception is Eden/Tainted Eden, who will have different stats and passive starting items.
When used for the first time, Esau Jr. immediately gains random items equal to the number of items the original character had.
Each new item will come from the same item pool the respective original item was taken from, similarly to the D4.
If an inactive character has an eternal heart when the active character goes to a new floor, the eternal heart will turn into a heart container when that character becomes active again.
If the original character doesn't fly and you swap to it on some part of the room just reachable by flying (i.e. beyond some water), you won't be able to leave (unless you can swap back to Esau Jr. or teleport by any means), effectively going into a softlock. This situation can be worked around by exiting and continuing the game, as the character appears next to a door.
Bethany / Keeper / Tainted Keeper: Esau Jr. will have one heart container, but can still gain heart containers up to the character's normal cap.
In the case of Bethany, Soul Hearts will still be used as soul charges instead of health.
In the case of Luna, the character that has the item must also be active going into Secret Rooms for them to spawn beams of light. However, either character can then enter the beam of light to gain tears and soul hearts.
Book of Virtues: Spawns a black wisp when used to turn into Esau Jr. that is otherwise identical to a normal one; turning back into the original character destroys the wisp. All other wisps are retained between forms.
Car Battery: Esau Jr. will only activate once, allowing it to be used normally.
Chaos / Sacred Orb / TMTRAINER: Will affect the items given to Esau Jr. when used for the first time, but will not affect subsequent uses. Item generation is only affected if the character that has it is currently active.
Eden's Blessing: The extra item given from turning into Esau Jr. for the first time will be different from the one given by Eden's Blessing.
Empty Heart / Missing No.: If the character that has the item is inactive going to a new floor, it will activate for them immediately when they become active.
The Forgotten: If used as The Forgotten, Esau Jr. will have three empty bone hearts and the bone club, and cannot gain soul hearts. Both forms will also be unable to turn into The Soul. If used as The Soul, Esau Jr. will not be bound to The Forgotten's Body. If used again to turn back into The Soul, The Forgotten's Body will also be summoned alongside them, but the player cannot turn back into them, preventing them from moving between rooms and causing a softlock.
Genesis: Using Genesis as Esau Jr. will revert back to the original character.
Lemegeton: Wisps will be retained between forms. For the character that doesn't have Lemegeton, the wisps effects are only updated when one of them is destroyed.
Lost Soul: The soul respawns every time the character that has Lost Soul becomes active. If it's the first time Lost Soul spawned after going to a new floor, it will give a reward, regardless of whether or not it died on the previous floor.
Panic Button: Activates Esau Jr. before the active character takes damage, redirecting the damage to the inactive character. The item's charge is used in the process.
Schoolbag: The extra active item will continue to be held by the character that has Schoolbag. However, it cannot gain charges through any means until the character becomes active again.
Tainted Cain: The items given to Esau Jr. on the first use are based on the item pool of the first room Tainted Cain gained an item from. Other abilities are unchanged.
Tainted Eden: Taking damage will turn Esau Jr. back into Tainted Eden, along with all of its other effects. Tainted Eden's health before transforming into Esau Jr. will not be recovered.
Tainted Jacob: Esau Jr. takes half a heart of contact damage from Dark Esau instead of turning into a ghost. Dark Esau's death explosion also does not affect him.
Tainted Magdalene: Esau Jr.'s starting black hearts will quickly drain down to half a heart; he must obtain heart containers while in this form to have any survivability.
PC J1KG XYFQ (First floor treasure room)
If used as Jacob and Esau, the transformation behaves very erratically, breaking the HUD by hiding either Jacob's or Esau's elements depending on which character uses it. Quitting a run where Esau Jr. has been used cannot be resumed without crashing the game.)
If used as Tainted Isaac, any items Esau Jr. gains on its first use do not count toward the item limit.
Using this item as Tainted Lazarus replaces both versions' spacebar items with it, and will re-replace any other spacebar items if one version keeps Esau Jr and the other exchanges it for a different spacebar item on next use; also, instead of swapping to Esau, they will swap between the two versions of Tainted Lazarus on use.
Esau Jr. can sometimes start with Spin to Win as a passive instead of an active item. It behaves like a regular orbital item in this form and cannot be used for increasing orbital rotation speed or Isaac's movement speed. The item will show up in the pause screen item collection but not the item collection HUD.
Tainted Forgotten can turn into Esau Jr. without gaining the ability to move, and turning back removes Tainted Soul, making it impossible to continue. Trying to quit and resume the game causes a crash.
If Esau Jr. is overcharged (using The Battery, Bethany/Tainted Bethany's soul/blood charges, a Battery Bum, or a Mega Battery), using it has a chance to cause the character transformed into to play at double speed. This includes having doubled speed and tears, halved invulnerability frames after taking damage, and halved duration for Temporary buffs. This effect can stack additively if Esau Jr. is overcharged before using it again. If this effect is caused by using soul/blood charges, all the charges will be consumed and the effect will stack up to the number of charges consumed. If this effect does not occur on use, the character's play speed is set back to their normal amount.
If used while a Purgatory ghost is onscreen, the game will crash.
If the debug command listcollectibles is used while transformed into Esau Jr, the game crashes.
If one character has Deep Pockets, swapping to the other character then collecting or spending coins will cap the number of coins at 99.