Eve

Eve is a character unlocked by beating two floors in a row without collecting any heart pickups. Eve starts with two Red Heart containers, Whore of Babylon, and the Dead Bird.

Eve will also start with the Razor Blade upon donating 439 coins to the Greed Donation Machine.
- However, if the Razor Blade has not been unlocked by defeating Satan with Eve, she will not start with the Razor Blade until both conditions have been fulfilled.

Notes

Unlike other characters, Eve's Whore of Babylon's effect activates with one remaining Red Heart, instead of half a heart. Additionally, Eve's 0.75x damage multiplier will change to 1.00x as long as Whore of Babylon is active.
Eve has a 16.25% chance instead of the usual 10% for any nonspecific Heart drop to turn into a Soul Heart.
When unlocking Eve, gaining Red Heart containers or using Yum Heart does not count as picking up heart pickups. However, the use of Pyromaniac does count.
Upon donating 439 coins to the Greed Donation Machine, Eve will start with the Razor Blade. This allows Eve to get to her Whore of Babylon while also granting the damage-up of the Razor Blade, and without lowering the Devil deal chance.
Eve works well when she's low on health; using Devil Beggars and Blood Donation Machines are good ways to activate Whore of Babylon.
Curse Rooms are generally useful rooms to find at the beginning of a run. Without any health upgrades, they will lower Eve's health into her Whore of Babylon state, and can potentially hold useful items and/or Soul/Black Hearts to protect Eve after transforming. If not already active, be sure to damage yourself upon the Curse Room door before accepting Soul or Black Hearts to activate the Whore of Babylon state.

Birthright: Whore of Babylon is active regardless of health and Dead Bird is active without taking damage. The effect will not occur if the corresponding item is removed or rerolled from Eve's inventory.

Items that take Red Heart containers away typically do so as a cost to counterbalance its potent benefits; for Eve, however, this supposed cost will likely be beneficial due to Whore of Babylon. Such items include Dead Cat, Guppy's Paw, and Abaddon.
Following the same logic, items that make deals with the Devil more likely will allow Eve to trade her Red Heart containers away more reliably, making it safer to gain health for the sole purpose of spending it in a Devil Room.
As Eve will prefer having little to no Red Hearts, she will need alternative ways to stay alive. As such, items that are conducive to spawning Soul Hearts or Black Hearts will be instrumental for Eve. Such items include Dark Bum, Book of Revelations, Satanic Bible, Mitre, and many more.
The Polaroid works very well with the Razor Blade, when Eve is at half-a-Red-Heart or less. During the period of invincibility after taking damage, the player can spam the Razor Blade quite a few times and vastly increase Eve's damage with no penalty.
Items that activate with low health such as Dark Prince's Crown work well with Eve.
If the player has unlocked Eve’s starting Razor Blade, then it is always beneficial to take HP ups and items that heal, so long as the player remembers to use it in rooms with enemies in them until Eve only has one Red Heart container

Any items that give Eve Red Heart containers are potentially bad since it could deactivate the Whore of Babylon effect. However, because Red Heart containers are very useful in deals with the Devil, it could also benefit Eve greatly to hold on to the containers temporarily. Thus, an understanding of Devil Room spawning occurrence and chance is essential in deciding whether or not to take such items.
However, if the player has unlocked Eve’s starting Razor Blade, then it is always beneficial to take HP ups, so long as the player remembers to use it in rooms with enemies in them until Eve only has one Red Heart container
Items that heal Red Heart containers (such as Placenta, Charm of the Vampire, and the Tick) can (and oftentimes inevitably will) cancel the effect of Whore of Babylon, if Eve has more than one Red Heart container.
Items that attract pick-ups (such as Magneto or Super Magnet) can force Eve into taking Red Hearts, which will cancel the Whore of Babylon effect.
Child's Heart causes more Red Hearts to appear, something which is generally undesirable in an Eve run without Dark Bum, and can make more useful pickups such as bombs or keys less likely to appear.

Eve was a playable character in the original The Binding of Isaac. She was added in the Halloween Update.
- In the original, Eve was unlocked by making two or more Devil deals in one run. This unlock method was given to Azazel in Rebirth.
According to biblical theology, Eve was the first woman on earth, born of Adam's rib in the book of Genesis.
In the original game, Eve had a unique effect in that she would leave a bloodstain when entering a room at half a heart remaining instead of a urine stain. This was thought to be a reference to menstruation, as biblically menstruation was a curse upon Eve given by God. This feature was initially (intentionally or unintentionally) left out in Rebirth, but was later re-added in Booster Pack #5.
Eve was confirmed to be Edmund McMillen's favorite character from The Binding of Isaac in a Twitter AMA.
Eve's locked icon shows her bound in chains.
- She shares this with Magdalene, Cain, Samson, Lazarus, Bethany, and Jacob and Esau.