Every time they take damage, all of their stats, items (passive and active), their trinket and their currently-held card or pill get re-rolled. Items are guaranteed to re-roll into other items from the item pool they were obtained from.
The re-rolls will easily remove health, but not restore it. This makes it risky to focus solely on Red Hearts or Soul Hearts, and a balanced approach is overall safer.
Powerful transformations such as Guppy are difficult to attain, as transformations are removed if the appropriate items are re-rolled, and will not be restored if the items are re-rolled back into (only by manually picking up the appropriate items will re-enable the transformation).
Shops are worth investigating, as they are the only way to obtain Birthright. Birthright cannot be obtained by re-rolling into it; it must be bought the conventional way.
If Gulp! pills or a Smelter are available, absorbing trinkets makes them permanent. If possible, they should be discarded to avoid losing them to a re-roll while looking for better trinkets.
If Tainted Eden re-rolls while holding two trinkets (such as from Mom's Purse or Belly Button), the trinket in the second slot is dropped rather than re-rolled.
Each item is always re-rolled into the same item (at least within a given seed and floor). For instance, reverting a re-roll with Glowing Hour Glass then getting hit will always give Tainted Eden their Glowing Hour Glass back.
Since Cards and Pills are re-rolled on hit, Tainted Eden should pick up 0 - The Fool before the fight with Mom and drop it at the beginning of the battle, so as to not lose it if they take damage, if they intend to use it to open A Strange Door.
Missing No. will not be re-rolled upon taking damage, but can be re-rolled into.
Birthright will prevent the currently held active item from re-rolling. However, if it is replaced, active items (including the original) will continue to be re-rolled.
There is no guarantee of consistency to Tainted Eden's health; While acquiring Soul or Black Hearts may be a good idea because they cannot be rerolled away, there is a chance that Tainted Eden rerolls into a build with massive amounts of red health, causing the overflowing Soul health to irrevocably disappear.
Obtaining Birthright may not be in Tainted Eden's best interest if the items they have acquired so far are of low quality. Deliberately taking damage, if it can be afforded, to re-roll into better items, and then taking Birthright, should be considered.
Gaining Heart Containers from sources other than passive items, such as Health Up pills or Eternal Hearts is beneficial, because those changes are not affected by Tainted Eden's mechanic.
More than other characters, Tainted Eden benefits from quantity over quality of individual items since the items are likely to get rerolled several times anyway. Obtaining as many passive items as possible means more items to re-roll and more chances for useful synergies.
Getting items from more consistently positive item pools can make a big difference. In particular, Angel Room items and Planetarium items are high-quality on average.
The Library can be powerful for Tainted Eden. If Book of Virtues is acquired from a Library, it cannot be re-rolled away due to how Book of Virtues is the only passive item in the Library item pool.
Holding NO! when taking a hit will re-roll Tainted Eden's active item into a passive item, which may or may not be desired.
Plenty of trinkets don't need to be held constantly to benefit from their effects, so dropping them when they aren't needed is often advisable. In particular, trinkets that affect level generation like Bloody Crown, Telescope Lens and Fragmented Card can be dropped in the starting room of each floor, and only returned to once ready to proceed to the next.
After rerolling everything, check if rocks are destroyed when walking into them. Several effects grant the ability to walk into rocks to destroy them, such as Thunder Thighs or Leo.
While "good items" and "bad items" are less of a factor of choice than for other characters since they are frequently re-rolled, there are some that should be of special notice if found or re-rolled into.
Abyss: Can be rerolled, but the locusts are permanent.
Birthright: Prevents items acquired before Birthright was picked up from being re-rolled. This includes normal re-rolls from items like D4 and Missing No., not only the damage-triggered re-rolls.
If Birthright is discovered in a shop it might be worth taking damage a few times to try to reroll a powerful synergy before locking it in with Birthright.
Converter: Grants Red Heart Containers which won't be rerolled away, significantly improving Eden's effective health for the rest of the run.
Esau Jr.: The ability to fly gained when transforming into Esau Jr. won't be rerolled away, even when losing this active item.
Glowing Hour Glass: If used after getting hurt in the same room, will return Tainted Eden to their previous build upon landing in the previous room. Should Tainted Eden get hurt again without ever dropping their previous active item, they will always re-roll it into Glowing Hour Glass.
Heartbreak: Will activate if rerolled into on otherwise fatal damage, which can save a run.
Holy Mantle: Triggering Holy Mantle will not trigger rerolls, helping Tainted Eden keep their current build.
Blanket: Serves as a weaker version of this, for boss battles only.
Lil Clot / Sumptorium: Summoned clots will persist after the items are rerolled away, acting as semi-permanent familiars until they die.
Prayer Card: If retained long enough to be used a few times, Tainted Eden will be able to gain significant permanent Health, which is hard to come by for them.
Panic Button: Because self-damaging items do not trigger rerolls, a strategy can be to hold such a self-damaging active such as Razor Blade or IV Bag in conjunction with this trinket to protect Tainted Eden from rerolls and even slightly benefit from receiving damage.
Purgatory: The effect will remain for the current room even if the item itself is re-rolled, providing a helpful boon during boss battles.
Red Stew: The temporary damage boost is retained even after the item is re-rolled, potentially acting as a semi-permanent item effect until the damage boost is lost completely.
The Swarm: The orbital flies granted do not disappear if the item is re-rolled.
TMTRAINER: All items picked up after an item re-rolls into TMTRAINER will become glitched items. Glitched items will not reroll anymore and will frequently be detrimental with their completely random effects.
Brittle Bones: Removes all Red Heart Containers but grants 6 Bone Hearts. Can be very strong if taken early or if low on Red Heart Containers. On the other hand, if Tainted Eden has a lot of permanent Red Heart Containers from Eternal Hearts and Health Up pills, it could be detrimental.
Chaos: When re-rolled into, any item appearing before Chaos in the item list (that is, older items) will re-roll into items from the correct pool, and items appearing after Chaos (that is, younger items) will be from random item pools. Triggering another re-roll will once again replace all items with those from the correct pool they were in before Chaos shuffled them.
If items are picked up after obtaining Chaos, triggering a re-roll will result in those items being replaced with another item from the pool of the item itself, not from the item pool of where they were obtained (i.e. a Devil Room item taken from a Treasure Room will re-roll into Devil Room items thenceforth).
Guppy's Paw: Converts one Red Heart Container into three Soul Hearts. Obtained early enough in a run, it allows for Heart Containers that are granted by passive to be converted into Soul Hearts that can not be re-rolled.
Satanic Bible: Provides Black Hearts which are a relatively stable form of health for Tainted Eden. However, using it will also replace the floor's boss item with a Devil Room item.
Tainted Eden's transition screen portrait, animated as in game.
Tainted Eden's locked character icon, depicting a horizontally glitched closet door
The achievement for unlocking Tainted Eden is "The Capricious", a possible name for the character.
"Capricious" means "given to sudden and unaccountable changes of mood or behavior," fitting Tainted Eden's concept of complete randomness.
Tainted Eden's hairstyle will change every time they are hit.
On the stage transition screen, their body glitches in a unique animation. Prior to Version 1.7.5, their portrait did not appear whatsoever.
In-development, Tainted Eden's effect was a full reroll upon acquiring a new item that excluded the item triggering the reroll rather than having the reroll be triggered by damage. This was changed because in testing players would simply stop picking up items once they found something good enough to be worth keeping.
Oddly enough, Tainted Eden unlocks XXI - The World?, associated with stagnation and inertia, this in relation of unfinished goals, lack of motivation and roadblocks, as any builds Tainted Eden makes will be lost in a hit, causing runs to feel meaningless and take free hits hoping for a better build or require absolute perfection to keep a strong build.
Tainted Eden consumes Eden tokens at an unpredictable rate: sometimes only one is removed, sometimes two, and sometimes more. The exact reasons and conditions are unknown.
If Tainted Eden is firing Technology 2 when damage triggers a re-roll, sometimes the laser persists firing in the last-fired direction until leaving the room, even picking up new synergies as other items reroll.
If the active item is re-rolled into Alabaster Box it will be fully charged instead of discharged as usual.
If Tainted Eden is resurrected during a Co-op game after dying, their graphics will break, cycling between Isaac's head and body.
If Tainted Eden first looks at an item on a floor (such as treasure room items, shop items, golden chest item drops, etc), but does not take them until after a Devil / Angel deal room has been taken, those items will be re-rolled into the respective deal room item pool.
If Tainted Eden rerolls into modded items that must be unlocked to be acquired, and they are not unlocked, the item will be rerolled into nothing, and you will lose an item. This can also cause crashes.