Stan is Stan, he was just never human in the first place (hence the name of the au HA). Stan is a Fae, a changeling slipped into the crib. Unfortunately, Stan has no knowledge of this (sometimes experiencing strange things, but thinking them as normal); only discovering what he truly is after Ford is lost to the portal.
The ‘Taking the Deal’ took place about a few months after Stan loses Ford. Stan makes the deal with an ancient Fae that has claimed the Gravity falls forest as it territory called ‘The Many Sights aka MS or Milly’. After Stan ventured into the deepest part of the forest dubbed ‘The Deep’ trying to escape some old enemies of his that have tracked him down (they do not come out of the forest alive >:3c) and he gets gravely injured.
He discovers he was never human to begin with and has a breakdown, he encounters MS. MS and some forest residents have ill intentions towards Ford for some things he did.
Stan makes a deal with MS to repay for Fords harms to the forest, becoming like a step in keeper/guardian to the forest while MS takes a nap.
Stan stability of keeping his human form is emotional and mentally based. As long as he thinks himself as human it’s pretty simple, believing he was human the first 30 years of his life it came easy. He can control the shifting but as easily when loses focus he loses control. like when he finds out the truth, his forms starts slipping for the first time, glowing green eyes, he starts glowing from the inside ( at the start Stan only has 2 eyes aka 2 sights, and gains 4 more when becoming keeper of the forest, ill get more on that another time)
This also happens when Ford returns and the possibility being kicked out of the shack at the end of summer, Stan starts having a slight identity crisis, pretended to be his brother for so long, trying to remember and get used to who he was before, Stanley is in the sense dead to the world’s eyes, so his form starts slipping, eyes glowing green, 4 more appearing (I actually want to draw this scenario out, hiding them at first with sunglasses etc.) though Stan has not yet fully turned into his full Fae form, fearing to be unable to return to his human form once he does.
Like I said, Stan is Stan and he loves his brother
Before finding out he’s a fae, Stan ate human food fine, though he started realising he has more energy when he is near natural habitats (forests, rivers, fields etc). After becoming aware, he doesn’t need to eat human food to survive, but he still does because of taste. Stan is able to just survive feeding on the forest energy.
However since he is the guardian of it, the forest states affects Stan’s health, if it’s in a good state, he’s good. However since he is connected to it, that now if he spends too long away from the forest (a couple of months) he starts to weaken and he has to return to it. However the type of Fae Stan is known for eating other supernatural creatures (and other Faes to become more powerful and gain the devoured Fae sights) and sometimes humans, though Stan hasn’t resorted to that…yet
Well young and just founded the blind society Fiddleford did get suspicious, but at first only because of Stan taking his brothers place. But he once accidentally walked upon with Stan and all his 6 eyes exposed. Fiddleford was terrified by the sight and shortly wiped his memories of that event while he was still the leader of the Blind Society. Though still years later and now with his memories slowly returning, he may fear for the beast with one eye, though there’s another one with 6 sights close by.
Fords starts to suspect something is off about his brother. At first, he brushes off for not seeing each other for decades, but after awhile starts noticing things. Like the radioactive waste barely had any effect on Stan, disappearing in the middle of the night, suddenly wearing sunglasses inside, claiming it’s a hangover even though he doesn’t smell of alcohol or one time the group encountered a gremgoblin, but left when Stan confronted it. Ford starts to investigate and coming to the conclusion that Stan is not what he seems, maybe possession or some kind shapeshifter.
Ford starts an entry with invisible ink about Stan in the journal. However, felt bad about even writing in the journal about Stanley. He rips out the page though still kept it (which actually neatly ties in with this comic where the twins find the page).
But eventually Ford finds out (one night he follows him). Stan tries to explain everything to Ford as much he can, and about the deal he made so Ford wouldn’t be in danger from some creatures. Ford doesn’t take it too well and it’s like you said yourself in the last ask, it does strain their relationship even more than before.
Por causa disto me ponho de joelhos perante o Pai de nosso Senhor Jesus Cristo,
Do qual toda a família nos céus e na terra toma o nome,
Para que, segundo as riquezas da sua glória, vos conceda que sejais corroborados com poder pelo seu Espírito no homem interior;
Para que Cristo habite pela fé nos vossos corações; a fim de, estando arraigados e fundados em amor,
Poderdes perfeitamente compreender, com todos os santos, qual seja a largura, e o comprimento, e a altura, e a profundidade,
E conhecer o amor de Cristo, que excede todo o entendimento, para que sejais cheios de toda a plenitude de Deus.
Ora, àquele que é poderoso para fazer tudo muito mais abundantemente além daquilo que pedimos ou pensamos, segundo o poder que em nós opera.
I am not silent on Thursdays I am thudnerous I am boisterous I am proud To wield a hammer Albeit not made of the cosmos And even though not all of Midgard Hears the rolling thunder Your devoted carries it in their hearts And with squared shoulders Until you destroy all of our attempts at being serious Because A Laugh Is More Powerful than a sneer