"Woah..." Mabel stared in wonder at the blue triangle.
"His name is Bill Cipher. See, I wasn't kidding when I said magic!" Dipper grinned.
"Hello, again, Pine, and I see you brought a friend!" Bill noticed the tree on Dipper's hat, and was remembering his name by that. He saw the star on Mabel's sweater, and instantly decided that was her name.* "and you must be Star!"
"No, I'm Mabel Pines, but you can call me whatever... You're so cute and small!" She was swept away by him, grinning at the very sight of him.
Dipper watched his sister with joy, glad to see her this way. "So, Bill, have you always been a triangle?" He got out a notepad. He hadn't just brought Mabel this time - he brought the notepad, and a couple of pens.
"Yeah, I think so. I mean, I can't remember when I was born." He shrugged. He didn't seem to remember anything beyond being in the cave, after a couple more questions.
"Ooh, ooh, my turn! Bill, can you do some magic?" asked Mabel excitedly.
For a moment nothing seemed to happen. But then Bill went from blue to purple, from purple to red, from red to orange, from orange to yellow, from yellow to green, and from green to blue.
The twins cheered at this feat.
"What else can you do?" asked Dipper, scribbling furiously on his notepad.
"Hmm..." Bill closed his eye, and raised his hands, and all the little rocks in the cave lifted up at once, before he slammed them back down, opening his eye.
They cheered again.
They visited him whenever they could. They'd go to the Shack after school, do their homework as quickly as possible, and run out into the forest to talk to Bill some more. The three of them began to share a deep bond. Bill was a mediator when they argued, and a confidant when they had secrets. They told him so much about the outside world - and he was ever so proud of them, but jealous, too. He wanted to leave, but he didn't know how.
"Hey, why can't you leave?" asked Dipper one day, sitting there, munching on a sandwich. "How do you know you can't?"
Bill considered this for a moment. "There's a barrier around this cave, keeping me here. I've tried to leave before. It's... It's impossible to get through, trust me." He looked down. "Can I have some of your sandwich?"
"How can you even eat? You don't have a mouth..." Dipper protested, but gave him an eighth of it.
"Hmm... Watch this." Bill closed his eye, and then it wasn't an eye. It was a mouth, and he used it to devour the sandwich in seconds.
"Wow... You're so cool, Bill..." Dipper smiled and hugged him.