Cain

Cain is a character unlocked by obtaining 55 or more coins at once in a run.

Cain starts with two Red Heart containers, one Key, and Lucky Foot.

Cain also starts with a Paper Clip trinket after 68 coins have been donated to the Greed Donation Machine.

Lucky Foot now changes bad pills into good pills (similar to PHD, although not telling the pill's effect).

Notes

Cain can only shoot tears from his right eye. This affects certain items.
- The R U a Wizard? pill will cause Cain to fire tears to his right exclusively.
- Blood Clot, Chemical Peel, and Stye have a 50% chance to do increased damage.
- Technology 2 will prevent him from firing normal tears, even tear-changing effects like Brimstone.
  - Technology 2 will no longer prevent him from firing normal tears and instead will cause him to also fire Technology lasers out of his right eye instead of his normal tears. These Technology shots have the same synergies and interactions with other items on top of the Technology 2 laser. Neptunus lets him fire tears normally. These lasers cannot be angled with momentum.

Strategy

Cain starts with slightly higher damage and speed than most characters, making him more agile and effective at the start. However, he has lower range and health than average, which means he has to put himself closer to danger and can take less punishment than most.
As mentioned above, Cain only shoots tears from his right eye. This makes it easier to aim as him, and lets Cain consistently peek around corners to shoot behind cover.
Cain's kit makes him the best character for hoarding items. His starting luck increases the chance of a drop after a room clear, and Lucky Foot boosts this even further by increasing his success for room drops and machines like Slot Machines, Fortune Telling Machines, Shell Games, etc.
If Paper Clip is unlocked, Cain can unlock every golden chest he finds, which significantly increases the chance for an item or a mass collection of pickups.
Cain's luck makes him a good candidate for tear effects, as many of them are more likely to trigger based on luck.
As he starts with only two hearts, Cain benefits from early health ups, and is not a very good candidate for Devil Rooms early on since they will significantly cut his endurance.
As Cain is likely to have a large collection of coins or keys after a few floors, it is wise to open up Shops, bomb Shopkeepers, donate ten coins to a Donation Machine, or donate to a Beggar till he pays out to maximize the chances of an Angel Room. Cain benefits more from them compared to Devil Rooms due to his low health.
Because Lucky Foot changes bad pills into good pills, pills are extremely effective for Cain. This makes Golden Pills and Echo Chamber great finds for Cain, as he will receive multiple guaranteed buffs from the pills.

Item Interactions

Abel's tears become red and deal 7.5 damage when held by Cain.
Birthright: Grants +1 luck. All floors that can have special rooms contain an upgraded Arcade.
Technology 2: Effectively acts like the Technology item.
Eye Drops: Since his left eye is covered, it's simply a tears up instead of the normal effect of +40% fire rate on the left eye.

Unlockable Achievements

Cain's Other Eye - Complete the Boss Rush as Cain.

Glass Baby - Defeat Mom's Heart on Hard mode as Cain.

A Bag of Bombs - Defeat Satan as Cain.

A Bag of Pennies - Defeat Isaac as Cain.

Abel - Defeat The Lamb as Cain.

Cain's Eye - Defeat ??? as Cain.

Evil Eye - Defeat Ultra Greed as Cain.

Sack of Sacks - Defeat Ultra Greedier as Cain.

D12 - Defeat Hush as Cain.

Green Baby - Defeat Mega Satan as Cain.

Silver Dollar - Defeat Delirium as Cain.

Guppy's Eye - Defeat Mother as Cain.

A Pound of Flesh - Defeat The Beast as Cain.

The Hoarder - Use Red Key (etc.) to open the hidden closet in Home as Cain.

Picky Baby - Earn all Hard mode Completion Marks as Cain.

Unlockable Starting Items

Donate 68 Coins to the Greed Donation Machine - Start with Paper Clip

Trivia

Cain appeared in the original Binding of Isaac and required the same coin count to unlock.
Cain is a reference to the Biblical story of Cain and Abel, in which Cain commits the first murder by killing Abel.
- Abel also appears as an item in Rebirth.
- Cain's special interaction with Abel is also a reference to this.
Glass Baby could be a reference to Cain having a glass eye.
Cain's locked icon shows him bound in chains.
- He shares this with Magdalene, Samson, Eve, Lazarus, Bethany, and Jacob and Esau.
The fact that he takes 55 coins to unlock may be a reference to how it is said that Jesus cast out 55 demons during his life.