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Repentance 1.7.9 on PS5.

What you start with

The mechanics that actually govern this character.

3 soul hearts, damage multiplier 1.05, speed 1.1. He can never gain red heart containers: anything that would grant a container, including an empty one, gives him the same number of soul hearts instead.
Starts with The Poop, a 1-room-charge active that drops a poop pile and knocks back nearby enemies, enemy shots and pickups. The pile can drop pickups when destroyed, blocks low-flying tears, and bridges gaps if bombed from the far side.
In Repentance he has an innate Lil Larva effect: destroying any poop spawns blue flies.
With no red health he gets a guaranteed Devil or Angel room on every eligible floor, and Devil deals cost him the red-heart price converted into soul hearts rather than a flat 3 soul hearts.

Soul hearts are his only resource and nothing except more soul hearts refills them. Play with a three-hit buffer mentality, closer to The Lost than to Isaac.
Use Poop as terrain. Drop it between yourself and a charger, and use it on retractable spikes while they are retracted or on Red Poops while they are disabled to destroy them permanently.
Farm the innate Lil Larva: place Poop every room without breaking the piles, clear through to a boss or a hard room, then break the accumulated poop all at once to dump a fly swarm into the fight.
Shop the Devil pool aggressively. Guaranteed deal rooms plus soul-heart pricing means a one-heart deal costs one soul heart, and several devil items give back more health than they take.
Bone Hearts arrive empty and cannot be refilled with red hearts, so treat them purely as extra hits of buffer rather than as a resource.
Beds give ??? three soul hearts, so a clean Bedroom is worth the detour.
Eternal Hearts still function: each acts as an extra half heart and turns into a full soul heart at the end of the floor, which makes Prayer Card genuinely good on him.

Strong on this character specifically, not just strong.

Ceremonial Robes: 2 soul hearts in a Devil room for 3 black hearts and +1 damage. Net positive health, always safe to take.
The Pact: 2 soul hearts for 2 black hearts plus +0.5 damage and +0.7 tears. Cost-neutral.
Abaddon in Repentance: 2 soul hearts for 2 black hearts plus +1.5 damage, +0.2 speed and fear tears. Also cost-neutral, and the black hearts arrive after the price is deducted so it can be bought at any health.
Empty Vessel: 1 soul heart for 2 black hearts, and because he never has red hearts it grants permanent flight plus a 10-second invulnerability shield every 40 seconds in any room with enemies.
Birthright: doubles the effect of every HP upgrade, so each container item gives him two soul hearts. Paired with Converter this loops into effectively unlimited health.
Dark Bum and Mitre: they convert the red hearts he cannot use into black hearts, blue spiders, and in Repentance Dark Bum can also drop cards, including V - The Hierophant and Dagaz.
Crown of Light: it re-activates on every room entry for him because it keys off having no red health, though a single hit still switches it off for that room.

Picks that are neutral or good elsewhere and bad here.

Yum Heart, Yuck Heart and every straight red-heart heal or full-health pill. They do literally nothing.
Guppy's Paw: it needs red containers to convert, so it is dead weight. Pick it up for Guppy transformation credit and swap it back out.
Little C.H.A.D. alone: it produces red hearts. It only becomes good alongside Dark Bum or Apple of Sodom.
Glass Cannon: the Repentance health drain no longer applies, but taking any damage while holding it makes the next hit in that room cost two full soul hearts, which is most of his health bar.
Thunder Thighs, Leo and the Stompy transformation: they destroy his poop the instant it is placed, killing both the wall and the blue-fly engine.

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

Ankh is a straight 1up for him, since it revives any character as ??? anyway.
The Poop is a 1-room charge, so any per-use trigger fires every single room. Dim Bulb in particular gives him a consistent stat boost.
Petrified Poop makes each Poop use a reliable pickup source, and it can drop itself from a Poop pile. With a battery loop it out-earns Book of Revelations for hearts and money.
Guaranteed deal rooms mean you can plan around Devil shopping: skip a mediocre treasure room item if the deal room might hold Brimstone, Mom's Knife or Dead Cat.
Satanic Bible in Repentance turns boss items into heart-priced devil deals, which for ??? are soul-heart priced and usually stronger than the boss item they replaced.

Lazarus' Rags and Judas' Shadow are specific traps for ???: dying converts you into Lazarus Risen or Black Judas, and every mark from that moment on counts for that character instead.
Mega Satan is one of his easier marks because his deal chance never degrades from red heart damage, so refusing every deal reliably produces the Angel Rooms that hold both Key Pieces.
His 1.05x damage multiplier is the weakest of the roster, so the timed marks (Boss Rush at 20 minutes, Hush at 30) depend on finding a real damage item rather than on his kit.

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