Monsterhearts 2
Monsterhearts is an indie RPG "about the messy lives of teenage monsters", designed by Avery Alder and published by Buried Without Ceremony in 2012. Monsterhearts 2 (in following: MH2) is an updated second edition of the game, published by Alder in 2017. Both versions are "Powered by the Apocalypse" – i.e. based on the Apocalypse World game system discussed previously, – but whereas AW (as well as Vampire, SIFRP, and DA) merely addressed sexuality and the use of erotic capital in social interactions, Monsterhearts is notable for being all about them.
Free resources: Reference sheets and core skins.
Pulling Strings
A key difference between MH2 and most other games in this survey is in its modeling of relationships between characters as a resource rather than as effectiveness. These terms, coined by The Forge community in the early 2000s, describe distinct ways to simulate a fictional character's capabilities in gameplay: "effectiveness" covers all qualities of a character that affect the chances and/or the extent of their attempted actions' success, while a "resource" is any abstract quantity that their player expends to power certain in-fiction actions or to mitigate in-fiction setbacks.
While some titles discussed above (GUMSHOE, Vampire, Numenera) don't factor existing agent-patient relationships into persuasion mechanics at all, others do so mostly in terms of effectiveness: in D&D, the valence of said relationship (positive or negative) shifts the outcome of the persuasion attempt up or down the gradient of success; in SW, it also modifies the likelihood of a successful persuasion; both SIFRP and AW grade relationships on a seven-point scale and make them asymmetric, but the Disposition Rating in the former is mainly a defensive effectiveness value, while Hx scores in the latter represent the patient's effectiveness in proactively interfering with the agent's influence.
MH2 instead models relationships as a resource known in the game as "Strings". Strings (p. 16) are an abstract representation of specific emotional and social leverage a PC (or an NPC, see p. 100) holds over another PC. By using the "Pulling Strings" basic move (p. 26), the agent player can expend one String they have on the patient to either offer them XP in exchange for their one-time compliance (similar to the "Seduce or Manipulate" move in AW, minus the element of chance), to give them a "Condition" (a persistent negative status effect that represents ill repute and makes the character more vulnerable, pp. 31-32), or to boost the effectiveness the agent's next move targeting the patient.
Other Basic Moves
While each playbook ("skin") has its own moves for gaining Strings on others, they are most commonly obtained via the "Turn Someone On" basic move (p. 18). When it is invoked, the agent attempts to convert their erotic capital (represented by their "Hot" stat) into social power over the patient: on a strong hit, they gain a String on them, and the patient player must choose from one of three predetermined reactions (including initiating an intimate liaison); on a weak hit, the patient player may choose between one of the reactions or giving the agent a String on them. As the designer notes, "this move is at the heart of how Monsterhearts understands sexuality, especially teen sexuality": neither the patient, nor their player can decide what turns them on (with the sole exception made for asexuality on p. 49), which other players can exploit to shift the power dynamic between their characters in their favor.
The opposite of Turning Someone On is the "Shut Someone Down" move (p. 20), wherein the agent attempts to take away some of the patient's power and confidence, usually in public. Their player then rolls with the their "Cold" stat, and on a strong hit, may choose either to erase one of the patient's Strings on the agent, to gain a String on the patient themselves (but only if the patient has no more Strings on them), or to give the patient a Condition; on a weak hit, they have the same options, but the patient gives them a Condition in return. The "Lash Out Physically" move (p. 22), wherein one character attempts to physically harm another, lets the target's player choose to gain a String on the attacker as one option on a weak hit (similar to how harming another PC raises their Hx towards the attacker in AW). Finally, the "Skirting Death" move (p. 30), which is invoked when a PC takes enough harm to die, lets them survive at the cost of losing all Strings they had on everyone else or else giving in completely to their innate monstrous nature ("Darkest Self").
We observe that because Strings can only be gained on the PCs (by PCs and NPCs), the system is uniquely geared towards "PC agent, PC patient" configuration, with the GM and their characters playing more of a second fiddle. It preserves player agency by giving Strings very specific gameplay effects that never impose on their free will, while explicitly justifying their lack of agency in who gains Strings on them with the unchecked nature of emerging teen sexuality.