Cain

Repentance 1.7.9 on PS5.

What you start with

The mechanics that actually govern this character.

2 red heart containers, 1 key, damage multiplier 1.20, speed 1.3, range 4.5 against Isaac's 6.5. Fast and hard-hitting, short-ranged and fragile.
Lucky Foot: +1 luck, better room-clear drops and machine payouts independent of the luck stat, and in Repentance it converts every bad pill into its good counterpart without identifying it.
He fires from his right eye only. That lets him peek around corners, and it means Blood Clot, Stye and Chemical Peel each have a 50% chance to land their bonus on his only shot. Eye Drops becomes a plain tears up because he has no left eye.
Paper Clip trinket (68 coins donated to the Greed Donation Machine) opens Golden Chests for free, so his keys go to shops and Key Master instead.

Take and swallow every pill you find. With Lucky Foot there is no such thing as a bad pill for Cain, so unknown pills are a free resource stream instead of a gamble.
With only 2 containers, floor 1-2 Devil deals usually leave him one hit from death. Cain is an Angel Room character.
Push Angel odds using the resources he actually accumulates: blow up a beggar (+35% deal chance), blow up the Shopkeeper (+10%), pay a beggar until he pays out, and donate ten coins to a Donation Machine.
His 4.5 range is the stat that makes early floors feel bad. A range item (Mom's Heels, Mom's Underwear) or a tear replacement matters more than a flat damage up in Chapter 1.
Use 1.3 speed to loop rooms rather than trade hits. Every hitless floor is worth +99% deal door chance, which is more valuable to him than to anyone with a bigger health pool.
Play the machines. Lucky Foot directly raises payout odds on Shell Games, Slot Machines, Fortune Telling Machines and Confessionals, and on Hard mode it lowers beggar payout thresholds from three donations to two.

Strong on this character specifically, not just strong.

Birthright: +1 luck, and every floor that can hold special rooms gets an upgraded Arcade. That is a permanent supply of machines Lucky Foot is already biased in your favour on.
Technology 2: in Repentance it no longer blocks his normal tears. Instead he fires Technology lasers out of his right eye alongside his tears, so it functions as a free Technology with full synergies.
Abel: while Cain holds it, Abel's tears turn red and deal 7.5 damage, far above a normal familiar's output.
Perfection: +10 luck, the biggest single luck source in the game, and it drops free after three hitless floors, which 1.3 speed makes realistic.
Luck-scaling tear items stack hard on him: Sacrificial Dagger, Tough Love, Little Horn and Euthanasia all convert his luck advantage into damage.
Blood Clot, Stye or Chemical Peel: each has a 50% chance to buff the one eye he actually shoots from, so half his shots get the bonus.
Any range up. He is the fastest of the classic roster and the shortest-ranged, so range is his single most cost-effective stat.

Picks that are neutral or good elsewhere and bad here.

Two-heart Devil deals before Depths. With two containers, one deal puts him at a single heart with Chapter 4 still ahead.
False PHD: it makes pills able to roll bad again, which undoes the entire reason Cain's pill economy works.
Number One and other range downs. At 4.5 base range his tears already die before they reach the far side of a room.
Rerolling Lucky Foot away with a D4, D100 or Dice Room. You lose the pill conversion, the drop bias and the machine bias, which is most of the character.

Verified against 1.7.9 on console — anything patched, PC-only or Repentance+ only is called out rather than listed.

Golden Chests cost nothing with Paper Clip, so every key can be spent on shop restocks and Key Master instead.
Blowing up a beggar is worth +35% to the deal door and blowing up a Shopkeeper another +10%. Cain generates enough bombs and coins to do both on most floors.
Devil Beggars, Blood Donation Machines, Curse Room doors, Sacrifice Room spikes and Bad Trip pills all count as self-damage, so they do not reduce his deal chance and do not destroy Perfection.
With Birthright, the guaranteed upgraded Arcade on every eligible floor plus Lucky Foot's machine bias makes Slot Machines a reliable source of Pretty Fly orbitals, which is real defence for a 2-container character.

Boss Rush is his easiest mark: 1.3 speed clears floors fast enough to kill Mom inside 20 minutes with no special items at all.
The Dark Room route (Satan, then The Lamb) is riskier for him than the Chest route (Isaac, then ???), because Sheol's Devil deals cost containers he cannot spare and every Chapter 4 hit is a full heart.
Angel-route play is also Mega Satan prep, since both Key Pieces come only from angel statues once you refuse every Devil deal.

Open the tracker for the ordered run queue, or how to unlock characters if this one is still locked.