8 Ball

 is a demon and an interdimensional criminal, from an unknown dimension, that appears during the events of Weirdmageddon.

Background
Eons before the events of the series 8 Ball indulged in criminal activities before being called together by Bill Cipher, along with Amorphous Shape, Hectorgon, Keyhole, Kryptos, Pacifier, Teeth, and Xanthar.

Season 2
In "The Last Mabelcorn," the arrival of 8 Ball along with its demon comrades into Gravity Falls' dimension is foreshadowed during the episode's cold open after Ford receives Bill's warning that Weirdmageddon is coming, 8 Ball's arms are briefly seen on an image of a wheel with symbols. Near the end of the episode, in a flashback, when Ford confronts Bill for lying to him, 8 Ball's laughing silhouette is seen in the representation of another dimension projected in the Mindscape.

In "Weirdmageddon Part 1," during the cold open, Bill introduces 8 Ball with the rest of his demon friends to the residents of Gravity Falls. Afterwards, on Main Street, 8 Ball and the other demons watch Bill turn Ford into a golden statue and taunt Dipper before burning the journals, after Bill burned the journals, he offers 8 Ball and Teeth to eat Dipper as a reward. However, Dipper managed to get away from them. Later in the episode, after Time Baby and the Time Police are vaporized 8 Ball and Teeth approach Bill to warn him that Dipper is still roaming free, and might try to free Mabel from her bubble. However, Bill isn't too worried, as he has Gideon on the case, so the party carries on.

Appearance
8 Ball is green, with a white chest and stomach, and has a muscular upper body. It has large ears, a prominent underbite and large teeth. 8 Ball's eyes are magic 8-balls which usually are pointed in different directions. The demon has cuffs around its right wrist and ankle, with a small segment of broken chain dangling from them.

Trivia

 * Robertryan Cory, the staff member responsible for 8 Ball's character design, had a lot of trouble coming up with 8 Ball's appearance.
 * This is further noted by the multiple character designs of the demon drawn by Cory.

8 Ball