Hello, /hgg/. Here's a massive character overview for the level-draining magical girl MSGK that I made in like 4 hours of an absolute writing bender as something of a proposal for your perusal. Honestly, this kind of thing helps me write individual scenes since I have something to reference, so don't take it for a "I am absolutely planning on writing all this and going positively fucking mental with writing until it's finished" kind of thing. It's just way too fun to lay down hypotheticals in a bulleted list when you've got a character idea in your brain.

>Name: Mina (looks like "Mine," which is what she'll make your XP. Also Japanese for "everyone," which is who she'll dominate, if given the chance)
>Initial Visual Design: Short (4' 11"), bratty-looking, human, MSGK, big-bottomed, wears a lot of pink in a basic, frilly magical girl outfit (Think Prisma Illya). Bright, blue eyes. Light hair of some kind (pink also?). Despite the MSGK, she looks very "earnest" in a sense, like she has a goal beyond just being a problem. Her main weapon is a magical girl staff tipped on one side with a venus symbol sporting a halo above it (the halo is like a cannon for blasts), while the other side is a ribbed, purple, dildo.
>Transformations: Optionally, gets bigger tits & ass the more she levels up, up to 100 cm of growth for each. Also has some temporary size-change magic.
>Personality: Bratty above all else, entitled, spunky, and energetic. Very apt to refer to things in video game terms. Morally flexible (kusogaki, kinda), but she also believes what she's doing is okay, because of silly, bratty justifications like, "I can use that power to beat villains way better than you can" and "Once I level up enough, I'll go beat and drain Lethice, too!" and things like that. Likes to show off, loves getting praised, loves wiping smug smirks off of others' faces (so she can be the smug one instead). Absolutely never learns her lessons when beaten, but maybe she'll reign it in if you can get on her good side? She takes great care of her "servants" (anyone who'll submit to her). Even if you befriend her, she'll still tease the shit out of you like, "are you sure you don't wanna give me all your levels so I can just beat up all the baddies for you? I'll let you be my personal chair for my big, sweaty butt whenever I get back~ <3" and such.
>Likes: Video games (she will largely fail to explain what the fuck a video game is to the champ), getting stronger, making others weaker, teasing people, sweets, big tits, big butts, sitting on faces, getting praised/worshipped, cute outfits, cute panties, girls, masochists who submit instantly, people who put up token resistance and keep making things worse for themselves
>Dislikes: Boys being way taller than her, huge muscles, losing, bugs, black coffee, furries ("You know, Daddy's knights used to skin those alive back home"), people who cry and make speeches and take things way too seriously

>History: Spoiled gamer princess in a modern fantasy world finds a creepy, floating, eldritch game console deep in the castle dungeon. Console merely asks on its screen, "Will you usurp the heroes?" and she inputs yes. It attempts to take her to Mareth and corrupt her through this contract (presumably so she can help manage problem cases like the champion of Ingnam), but it turns out she's already such a degenerate that it can't corrupt something that's already corrupt. Basically, she has a skill that says "ignore the effects of corruption by believing with your whole heart that you're completely pure." She arrives in Mareth with level-draining powers in the Magical Girl class (maybe she wakes up at that magical girl temple and got inspired by it). Notably, this is more "what she believes a magical girl is," than anything real. She just really likes magical girls and wanted to be one, so that's what she got styled like.
>How her powers work: She passively emits a field around her that "codifies" reality into certain quantities that can be manipulated in unintended ways. Instead of "levels" being abstract quantities that merely measure your strength, levels and stats become real, physical quantities that can then be attacked directly and stolen much in the same way you could beat someone up and steal their panties. Basically, when she's around, life literally becomes like a video game or a TTRPG with a really biased GM. In theory, a sufficiently powerful mage could counter this esoteric magic, so long as Mina doesn't grow too powerful. Additionally, "important" people (so, the Champ and named NPCs as opposed to generic mobs) have much richer/more effective XP for her to drain (explains why she doesn't level up outside of her encounters with you).

>Prerequisites for the Champ to meet her: Reach Level 16 and visit the magical girl temple once. The level requirement is so that you have something to lose in terms of stats.
>How the Champ meets her (First Encounter): The champ meets her in the forest after she fled from the bog where she was first transported to. She's being chased by a pack of imps that are too powerful for her to beat while she fires ineffectual blasts back at them. The Champ can either intervene (begins questline) or watch the amusing sight (defer for later). If they intervene, Mina will notice the champ and run straight for them, at which point the player takes notice of whether her chest and/or ass look like they're "full of potential" or "just perfect the way they are" (toggles Mina's progressive breast/butt expansion). However, due to being distracted by cute loli tits/ass, Mina takes them by surprise, bowling them over and kissing them to drain about 100 XP from them so she can grow to Level 2 (this also diagetically introduces the draining mechanics to the player) and have her HP restored. She then gets up and fires her now-much-stronger blasts at the imps and totally beats them really easily. She thanks the player in a mocking tone of voice, teasing them for being so easy. The player asks what just happened, and Mina explains that she stole a bit of their power for herself, and that as "thanks," she'll let them go this time, but also that if she sees them again, she's gonna drain some more, since it's totally fair game.
>How her encounters progress: Her level can go up to 100, and each increment of 20 unlocks new encounters with her. Once she hits 100, she is basically unbeatable and will continue to get stronger and stronger as she keeps leveling up past the cap, though she won't get new scenes. Whatever she drains, she keeps for herself in future encounters, permanently. Her encounters are primarily battle encounters, though there will certainly also be non-violent encounters (probably depends on how much Mina likes the champ). Each loss against Mina in battle results in Level Drain, though Mina is "kind" enough to let the player decide how they'd like to get drained (usually).

>Level Drain Scenes:
Facesitting: Mina sits on the player's face and drains their XP through her butt. It drains a medium amount, but Mina really likes sitting on people's faces. High affection gain. If the player has a pussy, she'll use her staff's dildo tip to fuck it. If they have a dick, she'll give a footjob.
Kissing: Mina pins the player to a tree and kisses them, draining their XP through her lips. If the player's too tall, Mina will summon a magic platform of solid light to stand on (cute!). At this point, the player can accept being drained, which has medium affection gain, or they can attempt to shove Mina to the ground and run (chance is based on strength), which drastically lowers Mina's affection regardless of the outcome.
Sex: Mina either tribs her heart out on the champ if they have a pussy, or if they have a cock and no pussy, she'll hop on top cowgirl-style and drain them that way. EXTREMELY, extremely high XP drain with high affection gain (slightly higher for the tribbing, as Mina is a bisexual degenerate with a slight bend towards girls).
Breastfeed (requires the champ to look feminine): Mina uses a spell to make the champ's chest grow bigger and milkier if below a D-cup/not lactating and begin lactating XP-ladem milk. The player then has the choice to accept that, complain about it, or ask to be made even bigger. If they complain or ask for bigger, Mina will crank them up to H-cups, either mockingly or generously, respectively. Mina then sits on the player's lap and begins sucking down all that milk, draining a moderate amount of XP.
Spanking (Level 20 or above): If Mina really hates you or thinks you need to be punished, she'll restrain you by summoning rope around your limbs, bending you over her knee and spanking you (and having the dildo-end of her staff shoved up your pussy, if it exists). If you're butt's too small for her to enjoy spanking you, she'll use a spell to make it bigger. If it's NOT too small, but you complain about being spanked anyway, she'll expand your ass even fatter anyway to bully you. Extremely high XP drain, and then she gives you the chance to say you're sorry before you go. If you don't say you're sorry, her affection stays the same. If you do say you're sorry, extremely high affection gain. Low priority would be creating a "roleplaying" version of this scene where Mina's not actually mad at you.
Full-body Titfuck (Level 40 or above): Mina uses the inverse of her Giga Growth spell on you, shrinking you down to the size of a fairy and giving you a full-body titfuck, draining your XP through her breasts.
Quick Peck (Only available at maxed-out or near-maxed-out affection): Mina pulls you in for a quick peck and cutely tells you to get out of here "before she changes her mind" with a flirty tone of voice.

>Battle Skills:
(Passive Demerit) "Don't Look Down on Me!" (Starting Skill): Mina takes 1% more damage from taller opponents for every centimeter of height gap, up to 50% maximum. Effect halved for female opponents.
(Passive) Growing Girl (Starting Skill): If Mina levels up in battle, her HP is fully restored (but not her Lust). ALSO whenever she levels up (not just in battle), her butt and/or breasts get bigger, depending on the player's choices during their initial encounter with Mina.
Tantrum (Starting Skill, but only used if she can't deal damage for 2 attacks): Runs up to the Champ and batters them with flailing punches to the chest, dealing a random amount of damage from 5-10% of the player's max HP and 5-10 Lust.
Tsuntsun Tease (Starting Skill): Mocks the player for any of: a small dick, an oversized dick, oversized tits, flat tits, an oversized ass, or a flat ass. This deals high Lust damage. If the player has none of those, she can only call them stupid and taunt them before talking about how she's going to drain them dry, which deals Lust damage but doesn't debuff the player.
Heart Blast (Starting Skill): A basic magical girl blast that deals magic damage.
Super Heart Blast (Gained @ Level 20, replaces Heart Blast): A much-stronger magical girl blast, and it's a "smart" attack in that it does physical damage or magic damage depending on what would actually cause more damage.
Butt Block (ALSO Gained @ Level 20): Mina enters a counter state where she intercepts an incoming attack with her big, squishy butt. Her defenses go to zero, BUT any attack that hits her gets converted into healing and XP, in addition to granting her a buff that makes her next attack deal double damage.
Giga Growth (Gained @ Level 40): Mina turns 3x bigger (so, she's about 12-ish feet tall) for 3 turns, making her deal double damage and take half damage.
ASSert Dominance (ALSO gained @ Level 40): Mina grapples the player, using magic to summon purple, magical ropes which restrain the player's limbs. She then sits on their face, rubbing her butt into it. This deals high Lust damage per turn, as well as draining the player's XP into Mina every turn. Both effects get 20% stronger for every turn the player fails to break free. If Mina is under the effects of Giga Growth, this move's effects are doubled as well, and the duration gets extended by 1 turn for every turn the player is caught (this is mostly to make scripting easier, but it also makes Mina stronger, so that's good).
Draining Kiss (Gained @ Level 60): Mina gently smooches you on the lips, draining a tiny amount of the player's XP, as well as 1 stat at random for herself and dealing high Lust damage. She does this as an additional action every turn, but it's a low-accuracy attack, so keep your Agility high or Blind her (if she's blind, it will never hit).
(Passive) Overpowered Adaptability (Gained @ Level 80): Every turn that battle with her progresses, she gains 2% damage resistance and a 2% bonus to damage she inflicts for the rest of battle (stacking additively). Whenever you attack her, she gains another stack of these bonuses, too. This activates every single time she's hit, even from moves or effects that have multiple hits. You're basically on an absolute timer of 50 turns (or more realistically, 25 turns). She ALSO gains 1 additional battle action.
Super Heart Storm (ALSO Gained @ Level 80): Mina summons myriad phantom clones of her staff floating in mid-air that pelt the champ with Super Heart Blasts. Attempts to hit up to 3 times. The first hit always hits, the second hit has 70% accuracy, and the third hit has 50% accuracy.
(Passive) Omni-Drain (Gained @ Level 100): Every successful attack Mina lands on you now drains 1 of each of your stats and inflicts 1-5 Lust damage. Also applies once per turn automatically during ASSert Dominance.
(Passive) "Look Down on Me All You Want; it Won't Help You~" (Gained @ Level 100): Cancels the effect of "Don't Look Down on Me!" and shrinks you 1 inch per turn during ASSert Dominance.

>Drops when Defeated: XP that scales with Mina's level (she doesn't lose anything, though) and "Magical Girl Panties," which you can huff for a bonus to your INT/AGI and big Fatigue restoration, in exchange for a huge spike in Lust, 1-3 Femininity gain, and height reduction of 1-3 inches if you're taller than Mina. Potentially can also be crafted into a pair of wearable magic panties with good stats/a good buff, too.

>Friendly Scenes:
Panty Entrapment (Level 40 or Above): Mina discovered that she can drain XP more efficiently at slower speeds (flashbacks to learning about Carnot Engines back in the day...) and uses the inverse of her Giga Growth spell on you to shrink you down to the size of a fairy, before stuffing you into her panties as she goes about her day. She goes into one of the settlements to shop for cute clothes all while passively draining you as you rub up against her magical girl pussy. There's like three opportunities to cum or hold it in, each climax giving her a burst of XP and making her appreciatively pat your back through her panties. Ultimately, she gets double however much XP you lose, instead of it being 1-to-1. She buys you a fun souvenir to give you to when you grow back, too. You also lose like 4 hours.
Drain-less Facesitting: Mina is happy to just sit on your face for like an hour, relaxing in a meadow or something as you both recover Fatigue.
(Open for suggestions for more of these)

>Other Ideas for me to explore/consider:
I think it'd be fun if she could kiss you and "learn" some of your skills by copying them, just to make her seem even more overpowered. This would then make her resistant/immune/heal against that skill when you use it on her, and she can use a "girlified" version of it against you, too. I don't really mind coming up with specifics, but that battle skill list up there was already stupidly indulgent anyway because big numbers make me hard.
Follower? Lover? Could be fun...
Her actually teaching you a magical girl skill or two once she gets powerful enough?
Some condition to remove her from the game if you don't like her (I'd hate to do that, but I should)
Bad Ends? I could see her draining your very soul if she is mad enough at you (so you live on inside her). You could also submit to her and become her lackey, where she then turns you into her magical girl sidekick with similar-but-inferior draining powers, too.
Different, cute panties depending on the day? Y'know how Jam in Guilty Gear has different panties for every day of the week? Yeah. Like that.
(Open for suggestions, of course)