Magdalene

Repentance 1.7.9 on PS5.

What you start with

The mechanics that actually govern this character.

4 red heart containers and 0.85 speed, the slowest of the starting roster. Damage multiplier is 1.00, so her entire advantage is the health pool.
Yum Heart: a 4-room-charge active that restores one full red heart. AAA Battery drops it to a 3-room charge.
With the Aprils Fool challenge cleared she starts with a Full Health pill, and that unlock also guarantees both Full Health and Health Up are in the run's pill rotation.
Birthright grants +1 filled red heart and raises her maximum health to 18 hearts.

Her speed, not her health, is the thing that kills her. Take the first speed item you see (The Belt alone is +0.3 speed) ahead of a marginal damage up, because at 0.85 she cannot dodge homing or fast projectiles.
Treat health as currency. Blood Donation Machines, Devil Beggars and Sacrifice Room spikes all cost half hearts you can refill with Yum Heart every four rooms.
The floor-1 angel play: at full health, with a charged Yum Heart and the Full Health pill, she can take seven Sacrifice Room spikes. The 6th sacrifice has a 33% chance to teleport her into the Angel Room and the 7th has a 33% chance to drop an Angel Room item outright.
Sacrifice Room damage counts as self-damage in Repentance, so spending health there does not reduce the Devil or Angel door chance for that floor.
Only red heart damage costs you the +99% hitless-floor deal bonus. Soul, black, bone and eternal heart damage does not, so once you have soul hearts stacked you can take hits without losing deal chance.
Her health lets her hold on-damage items that other characters cannot afford: Gimpy, The Virus, Razor Blade, Kamikaze! and IV Bag all get more uses out of her.

Strong on this character specifically, not just strong.

Any single speed up (The Belt, Belly Button-adjacent speed trinkets, Speed Ball): one item deletes her only real weakness.
Habit plus 9 Volt: Habit charges the active on damage and 9 Volt gives a partial charge per room, so a character that deliberately takes damage gets near-permanent Yum Heart uptime.
Sharp Plug with Habit and 9 Volt: pay health for charges, heal it back, and drain unlimited pickups out of Piggy Bank or Fanny Pack.
Guppy's Paw: converts her four containers into 12 soul hearts, the game maximum, which is more effective HP than she started with and keeps Devil deals affordable.
Blood Oath: the start-of-floor stab drains nearly all red health for roughly +0.15 x (hearts lost squared) damage. At 12 hearts that is about +20 damage, and Yum Heart refills afterwards.
The Wafer: halves every damage instance, which literally halves the cost of every Sacrifice Room spike and Blood Donation pull.
Birthright: +1 filled red heart and an 18-heart cap, which doubles down on the plan of spending health for items.

Picks that are neutral or good elsewhere and bad here.

Bucket of Lard: two containers is tempting, but the speed down on an 0.85-speed character means you start eating projectiles you used to dodge.
Dead Cat: sets her to one container and gives nine lives, which deletes her health economy and worsens her worst matchup at the same time.
Abaddon: converts her red containers into black hearts. She keeps the health total but loses the refillable resource that Yum Heart exists to top up.
Any further speed down. Taurus is the exception, because its speed gain over the course of a room eventually outpaces the initial penalty.
Spending all four containers on Devil deals in Chapter 1: you close Angel Rooms for the entire run, and the Sacrifice Room angel plan is her strongest opening.

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

A Bone Heart cannot break while it holds at least half a red heart, even from a full heart of damage, so a Bone Heart lets her pay a half red heart per Sacrifice Room use indefinitely.
Yum Heart charges per room, and waves count as rooms in Boss Rush and Greed mode, so it recharges every wave and gives effectively unlimited healing in both.
Blood Donation Machines and Devil Beggars do not reduce Devil room chance, so on a hitless floor she can drain herself into them and still keep the +99% bonus.
Perfection (+10 luck) is not destroyed by Sacrifice Room spikes, Blood Donation Machines, Devil Beggars, Curse Room doors or Bad Trip pills, all of which count as self-damage.

Her speed is the limiting factor for both timed marks: Boss Rush needs Mom dead in 20 minutes (25 from Mausoleum or Gehenna) and Blue Womb needs Mom's Heart in 30. The Strange Key trinket opens the Blue Womb passage regardless of the timer, and Mama Mega! opens both.
The Sacrifice Room angel plan doubles as Mega Satan prep: both Key Pieces come only from angel statues, and she can farm angel rooms with 6th and 7th sacrifices.
Mother and The Beast punish low speed hardest because all damage from Chapter 4 onward is one full heart per instance, but she also brings the largest health buffer of any starting character to absorb it.

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