I can safely say with absolute certainty, that this has become one of my favorite mods in Isaac, and my personal way of expressing interest is to come up with concepts for them that I frankly may put too much effort into given my lack of skill at modding. That said, since Seija has been part of the mod for about 2 years now, I thought I'd throw in a idea for a 4th addition to the Touhou cast.
I'm not calling it a suggestion because characters are harder to make then items by a long shot.


Remilia
The elder of the scarlet sisters, and the one that has effectively vanished from the limelight in Touhou in general since 2009, in spite of this though, the vagueness of the whole "Controlling Fate" power and her general characterization, including the fact that when really looking closer she's actually MORE violent then her little sister made her feel like she'd be an interesting addition to Isaac imo.


Traits

Vampiric Nature.
Suffers Stat decreases when exposed to Water, Carrying or using Bean items and when "Sun" effects trigger. But also gains more or greater stat increases when items or even curses involve Darkness or "Night" in some way. Has innate flight.


Twist of Fate; Your will is absolute. (Pocket Item: 12 charges)
Can be used at any time as long as it has one charge, what it does varies depending on the room you're in and how many charges it has. Example: Using it in an Item room will alter the potential Item/Items that appear in the Next Floor's item room, generally increasing the odds you'll get something of Higher Value. 3 charges for instance will remove all quality 0 items from the next floor's pool, while the max of 12 charges will guarantee you'll find Quality 4 items on the other side of that door.
Twist of Fate itself is part of the Devil and Curse Room Item pools and is quality 4.

Seija interaction:The effects of the item can backfire to Seija's detriment, regardless of how many charges it has. Example: Even at just 1 charge, it can force Quality 4 items into the next Item Room, which are considerably worse when Seija is grabbing them.


Stats
Health: 3 Red Hearts

Damage: 3.5 (1.10)

Tears: Deafult.

Shot Speed: 0.9

Range: 6.5

Speed: 1.2

Luck: 1


"2nd" Associated Item: Gungnir; Spear them Divinely (Active Item: 6 charges)
Readies Remilia's signature spell card "weapon", once fired, it launches a piercing, very high projectile in the direction of your choice, exploding into numerous tears upon impact with the wall.
Quality 3, found more commonly in Devil Rooms, and rarely in Item Rooms.


Birthright effect
Fate protects you; Grants Remilia a powerful "Soul" aura that repels enemies and projectiles.


Remilia, The Ravenous
Remilia, despite her childish demeanor, soundly reflects what you'd expect a typical vampire to be, a well dressed and (mostly) well-mannered noble with plenty of loyal servants, in other words, the image many vampires wish to appear to others as.
Her Tainted version on the other hand, embodies all of their Monstrous traits, the type of creature a vampire really is at their core, a ravenous fiend of the night that is ruled by their endless thirst for blood, sometimes degenerating into mindless rabid monsters if it isn't kept in check, which is exactly the creature Tainted Remilia has become.

In comparison to her normal version, Tainted Remilia appears far more like a savage ghoul than a refined vampire, her already pale skin has taken on an almost corpse like tone, her teeth and claws are longer and more pointed, her hat is gone and her hair has become longer and much more unkempt, the sclera of her eyes has turned pitch black, her normally clean white dress has been slightly torn and rendered a very dark red from the countless amount of blood that's been spilled on it during her feedings and even her shoes have been cast aside.
Any sense of rational thought has long since left her and all she thinks or cares about now is to satiate her hunger for blood.


Traits

Maddening Blood Thirst
(Tainted Remilia has a melee claw swipe instead of tears, and has a Blood gauge that acts as her "Hunger". It depletes over time and can be increased by running over puddles of blood left by slain enemies or hitting organic "non-poop" enemies with her pocket item. At certain thresholds she shifts to different states.)

100-75%; Satisfied. (Normal Stats, drinking blood at this point restores half a heart.) Appearance is "normal"

74-50% Hungry. (Normal Stats.)
T!Remilia's mouth contorts into a snarl and she visibly scowls.

49-25% Ravenous. (Increases all Stats, but the blood gauge starts depleting faster. If it falls down into "Starving" this state cannot be re-entered until she's restored all the way back to "Satisfied".)
T!Remilia's eyes shoot wide open to fully reveal her eyes and her mouth opens into a silent, animalistic wail that exposes her fangs.

24-0% Starving. (Decreases all Stats and health starts draining by half a heart every 15 seconds.)
T!Remilia's mouth closes and her eyes droop beneath her hair as she clutches her stomach.


Nosferatu; I Hunger... (Pocket/Active Item: 1 Charge (Refilling))
Performs a short leap in the direction the character is moving, if they collide with an organic enemy, they'll grab them and bite into them with a pair of vampire fangs, damaging them and restoring half-a-heart. If they collide with a poop or stone enemy, they will simply strike them. On Tainted Remilia, the bite effect increases her Blood Gauge and won't heal if she's below Satisfied.
Quality 4 and is rarely found in Devil or Curse rooms.

Seija interaction: Deals less damage and doesn't heal.


Stats
Health: 3 Red

Damage: 3.5 (1.25)

Tears: -0.7

Shot Speed: NA.

Range: Melee

Speed: 1.0


Associated Item
Hemophilia; BRING ME BLOOD! (Passive Item)
Grants a stacking increase to all stats for every "Blood" Item you have.
Quality 3, found in Curse Rooms or rarely in Red Chests.


Birthright Effect
Boundless Hunger; Permanently locks Tainted Remilia into her "Ravenous" State and stops her blood gauge from depleting.


Soul Stone
Soul of Remilia; (Paint them Scarlet)
Turns the character into a "Ravenous Vampire" for the current room, increasing their stats and replacing their tears with a close range claw swipe.



And... finished! Feel free to tell me just how crazy I am for actually bothering to type this all down, or maybe provide general feedback if you found these ramblings interesting.