I feel like if Dipper were ever reincarnated as a demon, he wouldn't fit in super well with the others. Yes, he's been raised to vie for power and step on everyone in his way using whatever means is necessary - it's the same toxic bizz as when he was a human, appealing to gender norms. He's tougher, scarier, more powerful (than ordinary humans, that is), but when it comes to asserting control - being Evil - he doesn't have it in him. Given enough time, I think he'd grow pretty vocal about leaving living things alone. NOT torturing organisms for the hell of it, or stealing people's souls, or conquering planets. Sure, he's a demon. That's no excuse to be a MONSTER.

It's a VERY unpopular opinion amongst neighboring demons, and rumor spreads fast about the Goody Two-Shoed Activist imp raining on everyone's blood-splattered parade, so much so that it makes it to Bill, who's immediately intrigued. Call it intuition, but only one soul's capable of overriding goddamn demon nature for some preachy bullshit about "Doing Good." Lucky for him, demons occupy the same plane of existence, so all it really takes to verify the guy is a snap of his fingers, and POOF! He's floating right next to him. Sure enough, Dipper's fashioned himself a new and improved demonic form, and it is lovely!

No one likes Dipper's kumbaya "Can't We All Just Get Along" ideology, but Bill's almost instantly smitten with the guy, whoever he is, so he's gotta be at least somewhat powerful. Demons take notice when the all-powerful Bill Cipher starts lending his time (and magic?) to some low-leveler like Dipper. Is he being blackmailed? Are they working together? No. Not possible. Bill doesn't "work" with anyone, save for whatever human catches his eye every few decades. Doesn't look to be doing him any benefit, either. The opposite, even. Lending power to a saint like Dipper only makes it harder to cause chaos, after all. Why would he actively go against his OWN best interest to cater some imp's? It's almost like he's. He's.

A henchmen.

(Bill's also 30% more affectionate the first month they reunite, because he still can't believe that his adorable little human husband came back as the same SPECIES as him! He'd never complain over having a sweet human to squeeze, but one with teeth and claws and cute pointy ears doesn't hurt).