He's faking it, Sans. (Undertale x Gravity Falls Crossover)

Chapter 2

So there was a quantum physicist in town. Interesting.

Bill leaned against his hands, watching with interest as the skeleton peeled open a bag of chips. He didn't carry himself like one. His posture tended to slack, with clothes stained and dirtied. He was small and adorable, in a weird way, not really like a scientist should be. Alphys and Sixer carried the look better.

But he solved the fake equation Bill set up. Which meant Sans was the man Bill wanted.

Bill watched him for the next few days, trying to figure out a decent plan to get the equation out of this Sans. Looks like the two old twins already warned him about the equation, which meant trying to strike a deal wouldn't work. Hold someone he cared about hostage? No, he probably wouldn't save them in exchange for the world, he seemed smart. So that meant Bill had to either trick him into somehow writing it down (unlikely), get him to make a deal (even more unlikely), or get Sans to trust Bill more than the twins.

Which meant, basically, get him to fall in love with him. Mortals were so easy to control through love, this would be the best option. Not the easiest, but the most secure. Get Sans to fall in love with him, break the barrier, then conquer the world. Maybe even keep him around. He seemed smart, and having a scientist on his team would be a major advantage in case any other problems arose their ugly heads.

But how would he go about making this skeleton fall in love with him?

The first thing that had to go would be his appearance. He couldn't just strut up into town as his demonic self, that'd ruin everything. And he had to get Sans head over heels in love with him, to the point where logic wouldn't hold a dime to his intense emotions. That kind of love. Because, with that love, Sans wouldn't mind giving up anything and everything to Bill, including that equation, so that'd be helpful.

So first, he'd focus on the body. Bill couldn't go into his mind without a handshake, Instead, he simply opted to browse Sans' computer when he left, finding his online profile and looking over preferences. Alright, he was into guys. A male human body. Seemed to swipe on the conventionally attractive men, so Bill went to work on that. Gave him the usual looks any girl or gay guy would drool over. Tall and broad, abs kinda guy. The type Mabel would probably be into. Oh, right, if Bill was going to go into the real world for good as this guy he'd have to worry about Mabel.

It's been awhile since he messed with Shooting Star, though, so that'd be fun. Maybe he could tease her with how gay he was, how this perfect body of a male would never be interested in a girl.

Mortals were weird in that way. 

It didn't take anytime to finish the body. Thick blonde hair, a decent complexion for mortals, it looked fine. He had to make himself a little more unique and special though, so he gave himself pure golden eyes. He needed Sans to be attracted to him, no one else, for this plan to work. Had to make himself special.

For personality, Sans didn't seem to be picky. That'd be an aspect that he would had to adapt to on the go. Learn what Sans liked, and adapt from there. But that only rose another problem for Bill to deal with. How exactly he would even be in Sans' life to begin with? He couldn't just strut up, that didn't seem to go over well in many mortal romance movies.

Luck deemed itself onto Bill's side as he found out Sans was looking for a roommate. His house had a spare bedroom, so he was trying to rent it out. Not many takers, especially since Gravity Falls wasn't exactly a perfect vacation spot.

Once his plan was fully developed, it became too easy. Bill knew the ins and outs of mortals, he's been tricking them for generations. He simply used his magic to add himself into their system, and created a rather bland backstory. Bill, short for Billy, born in... hm, what was a quite, simple state awhile away from Oregon? Idaho was comparable. Bill, short for Billy, born in Idaho and ready to move to Oregon. He saw Sans' thing online, and decided to pick Gravity Falls. Perfect.