First off, all characters have a max Ahegao that's equal to their max HP. The two values are completely separate aside from that however; if a character has 20 max HP, they'll have 20 max Ahegao as well, but if they take 15 damage, they're still sane if they take 6 Ahegao. Unlike HP, a character faces no extra risk for going into negative Ahegao, except that it will take more effort to bring them back to positive.
Any effect that restores HP can be used to restore the same amount of Ahegao instead. This must be either all HP or all Ahegao, it cannot be split. The decision is made by whoever creates the effect; the caster in the case of a spell, or whoever created the potion, scroll, etc. (For instance, if you find a normal potion of cure light wounds, it can only be used for HP; you can find a potion of cure light insanity, which does the same for Ahegao, but has no effect at all on HP.)
Pleasure and Losing Ahegao: Whenever a character is raped by a monster, they gain some Pleasure. Generally, characters start at 0 Pleasure, and lose 5 Pleasure (to a minimum of 0) every round, unless that character is being stimulated by something. The amount of Pleasure gained is usually equal to the number of tentacles on the girl when the monster spends an action to rape her, but some effects and monster abilities can raise or lower this (for instance, a mage might learn a spell that reduces how much Pleasure is gained, or a monster may have a ribbed tentacle that causes more Pleasure). Every time a girl is raped by a tentacle monster, she must make a Will save with a DC equal to her Pleasure. Failure means she orgasms, taking Ahegao Damage, and lowers her Pleasure by her total Will save. The total Ahegao Damage taken depends on the monster; weaker monsters only cause a few points, while stronger ones can quickly make a girl go mad after only a few massive orgasms.
Example: Terri is currently being held and pleasured by a monster, and has a Pleasure of 4. After three tentacles try to rape her again, her Pleasure increases to 7, and she makes a Will save. She succeeds with a 12, and faces no consequences. Next round, those same three tentacles try again, causing her to go to 10 Pleasure. She only rolls a 7 on her Will save, so she orgasms, taking some Ahegao damage, and her Pleasure drops down to 3.
A quick bit of reasoning for the changes:
First, keeping HP and Ahegao equal solved a few problems, such as how to deal with effects like Toughness that increased HP, or what to do for multiclass characters. I didn't like the idea that someone who got beat up a lot and then raped a little bit would go insane, however; I wanted them to be kept separate.
Calling the increasing Will save "Pleasure" means a few things, mostly that I can refer to it by name. A monster might release some lust pollen in the air that causes everyone to gain Pleasure and make them more susceptible to tentacle attacks, for instance. I liked the idea of building up to an orgasm and then taking the actual damage after a few rounds, but I didn't like that if the monster spent a while to build up a girl's pleasure and then raped her once, she'd orgasm and that would be it, so I decided that it would only go down by whatever her will save was. This could possibly mean a girl could build up a lot of pleasure and then have multiple orgasms quickly, if she's not careful.