Though he does not appear in the game or in the movies, the adult version of Marty's uncle Joey Baines is shown for the first time ever in the IDW comic adaptation of Citizen Brown. "Jailbird" Joey's adult likeness is based on Jason Hervey, who played Joey's older brother Milton Baines in Back to the Future (1985).
The trivia item below may give away important plot points.
Traits of the dropped character Tiff Tannen were incorporated into the character of alternate Jennifer. Though Tiff did not appear in the game, she was included in the IDW comic adaptation of the game, making her first appearance ever in the franchise.