Warning here; this is the story where I do add swearing but only because they're mature adults and it's unlikely that Bill would be so censored.
Dipper's face promised a slow and painful death to anyone who distracted him. He strapped his gun into a leg holster, pinned his badge to his belt, and headed out the door.
Bill was, of course, the one who intercepted him.
"What's got your gun all jammed, Pine Tree?" He asked playfully.
"Out of my way, Cipher, before I shoot you for obstruction of Justice."
Bill's eyebrows shot up. He learned new things about Dipper Pines every day.
"Woah, calm down there. I just want to know who killed your cat or whatever. Jeez, how unfriendly."
Dipper's face was dark. "None of your business!" He snaps angrily.
"Ah, except for the part where it is, in fact, my business. You see," he spread his hands widely. "You are my partner."
In the end, Bill went with Dipper on his 'errand', which ended at Mabel's house. Bill didn't know Mabel that well, but called her Shooting Star because of the earrings she wore.
As it would turn out, Mabel had gotten a new boyfriend and Dipper was there to scare him off with a badge flash and a pretty gun.
Or, at least, to test him. If he had done something in the past, he would likely ditch at the first sight of a cop. If he was innocent, he might stick around. Might.
Bill went in to watch the fun.
"-I don't care who you are, if you touch a hair on Mabel's head, I will personally take care of you. Are you listening?"
"Yes. I can see the headlines now; police brutality against an unarmed citizen in the presence of his partner and sister. Wonderful."
Dipper paused, stared at the kid, then nodded approvingly to Mabel.
"Good job this time, sis. Better than some of the previous, ah, mistakes."
Dipper turned on his heel with that, leaving Mabel spluttering indignantly behind him and the smirking boyfriend, Jayce, in the living room.
"Remind me never to piss you off, Pine tree."