Mr. What's-His-Face

Mr. What's-His-Face is a shadowy, face-stealing monster Pacifica summoned in an ill-advised attempt to get rid of facial wrinkles. He then went on to steal Mabel Pine's face and escape to Gravity Fall's paranormal black marketplace. Eventually, Pacifica and Dipper were able to get Mabel's face back and lead Mr. What's-His-Face back to Gravity Fall's, where he was frozen by Ford to be added in his bunker.

History
One day, Pacifica Northwest visited the Mystery Shack to show the Pines twins she has gotten her first wrinkle. She is stressed over it because her family's annual photo shoot is tomorrow that day. Pacifica says she needs Dipper's "creepy book" to fix it, but Dipper refuses to give it to her, not wanting to risk any danger. Pacifica seems to accept it and asks the twins if she can use their powder room. Pacifica in reality was lying and finds Journal 1, proceeding to summon Mr. What's-His-Face. She orders the monster to fix her wrinkle, but Mr. What's-His-Face convinces Pacifica to let him steal her face, only for Dipper and Mabel to barge in before he can do so. Mr. What's-His-Face then accidentally steals Mabel's face in a panic. Satisfied with getting Mabel's face, Mr. What's-His-Face puts Mabel's face in a burlap sack and teleports away. Mabel's faceless body reveals to Pacifica and Dipper that she can see out of the bag, and What's-His-Face is headed towards Main Street. Dipper and Pacifica then run off to pursue What's-His-Face and get Mabel's face back.

They find What's-His-Face just as he enters a door to a building. What's-His-Face taunts the two to try and catch him before entering the room and closing the door. When Dipper reopens the door, a brick wall has replaced the dark abyss What's-His-Face walked into. Dipper uses Quentin Trembley's President's Key to open the door where the two search for Mr. What's-His-Face in Gravity Falls' hidden paranormal black market.

After getting directions from Limby Jimmy, the two find Mr. What's-His-Face selling faces to nearby patrons. Dipper says they should follow him to find Mabel's face, but Pacifica spots a monster named Dr. Crowsfeet selling beauty pills and buys some from him. Angered by Pacifica's vain personality, Dipper yells at her to accept that she is human, only for the nearby monsters to overhear this and kidnap the two and have a human auction. Mr. What's-His-Face buys the two and tosses them in his burlap sack.

The two are dumped out of the sack and into Mr. What's-His-Face's home, with countless mounted faces on his wall, including Mabel's. Mr. What's-His-Face then steals Dipper's face and then morphs into Pacifica's dream face, but warns her that if she refuses his deal she will grow ugly and no one will love her. Pacifica refuses and throws the beauty pills at Mr. What's-His-Face, severely damaging his face. Pacifica then catch's Dipper and Mabel's faces in a basket and attaches Dipper's face to his body as the two run off, What's-His-Face in pursuit.

The trio manage to escape the Crawlspace and Ford is able to freeze Mr. What's-His-Face with a freeze gun, telling them that Mabel's faceless body showed him where they were. Ford then says he plans on adding the now frozen What's-His-Face to his bunker.

Trivia

 * His name is a play on the phrase "Mr. Whatshisface", which is used whenever someone can't remember someone else's name.