Tainted Bethany is unable to utilize Red Hearts as health and cannot obtain red heart containers, similar to ???. When picked up, red hearts instead are converted to a resource unique to Tainted Bethany called "blood charges" at the rate of one charge per half heart. The number of blood charges currently owned is listed along with the pickup counters. Blood charges can be consumed to use Tainted Bethany's activated item when the item isn't fully charged, at the rate of one blood charge per empty bar of charge.
Lemegeton has a 25% chance to pull from the current room's item pool when creating item wisps, and as such can be abused by Tainted Bethany to obtain large amounts of useful items by saving her blood charges until she enters a room such as Angel Room, Devil Room, Secret Room, Shop, Curse Room, or even a Planetarium.
Blended Hearts will count as red hearts and only contribute towards the red heart charge meter.
Maggy's Bow: Doubles blood charges gained when picking up red hearts.
Mega Battery: Both Tainted Bethany's held active item and Lemegeton are overcharged.
Red Key: With her blood charges, Tainted Bethany can explore more red rooms than other characters, allowing her to gain the benefits of red special rooms easily. In turn, this allows her to use Lemegeton to gain items from those item pools in addition to their normal contents.
Satanic Bible: This item is particularly well-suited for Tainted Bethany, as blood charges may be used to repeatedly activate Satanic Bible and potentially compensate for the price of the Devil Deal that spawns in place of the boss room item pedestal.
Activating Lemegeton in a boss room after activating the Satanic Bible on that floor will cause it to pull from the Devil Room item pool when creating wisps. This works even on bosses that don't normally drop a standard boss room item, such as Mom, Satan, or ???.
Lil' Battery: Same effect; active item will be charged first. If more charges than necessary for full charge are added, remaining charges will be lost, and not be added to Lemegeton. If picked up when active item is fully charged, charges will be added to Lemegeton.
The Jar: Hearts will only collect in The Jar after achieving 99 blood charges.
Red heart restoration: All items and effects that restore red hearts directly will instead add blood charges.
Full heal effects, however, do not give anything.
When Tainted Bethany dies, the orb embedded in her forehead stops glowing.
The achievement for unlocking Tainted Bethany is "The Zealot", a possible title for the character.
A zealot is someone with fanatical devotion to their faith or ideals.
This also correlates with the meaning of her card, V - The Hierophant? associated with lack of adaptability (zealotry to tradition), good understanding (Lemegeton) and weakness (worse items); Further shown with Lemegeton's links to judaism, the orthodox belief that fathered christianity.
Tainted Bethany's design is based off of the Exorcist and Necromancer enemies from The Binding of Isaac: Antibirth.
Tainted Bethany's locked character icon, depicting a closet door with a inverted cross and bloodstains on it.
Obtaining Polydactyly as Tainted Bethany will still allow you to hold two consumables alongside the book. However, if it is acquired through Lemegeton wisps, it could end with her losing the book completely if the wisp is dissipated, as picking up a consumable after losing Polydactyly could get rid of Lemegeton entirely.
Certain items that spawn blue flies in large quantities can cause wisps to disappear (further testing required).
Notched Axe can be used once fully depleted of regular charges, as long as Tainted Bethany has at least one blood charge remaining. Notched axe does not deplete blood charges, and needs to be reactivated every time it is used to break a rock or open a secret room.
Using Glowing Hour Glass and Sharp Plug while having at least 2 full hearts allows you to repeatedly use Glowing Hourglass and duplicate Lemegeton wisps allowing you to have infinite copies of said wisps.
Using Sacrificial Altar with item wisps will remove them without immediately removing the effects. The effects will be removed upon the next use of Lemegeton.