Giffany's creators were the various programmers of the game "Romance Academy 7." When programming the game the character Giffany eventually gained sentience. The programmers unsuccessfully attempted to delete her, and in response she "deleted" them.
Jeffrey Cannuck is a victim of the Blind Eye Society'smemory erasing. Based on where his memories are stored, it can be inferred that he knew notable information about Gravity Falls' supernatural side.
Mr. Northwest was a robber who lived some time between the 19th and 20th centuries. He is an ancestor of the modern Northwest family.
Soos' father is Soos Ramirez's deadbeat father. Due to the man's constant travels, he has not seen his son since 1994. Soos realized the extent of his father's neglect on his twelfth birthday, which led to a decade-long disdain for the date.
Soos' grandfather is Soos Ramirez and Reggie's late grandfather and Abuelita's former husband. According to her, he is "not [in heaven]." This implies that she thinks he's in Hell. 
Soos' mother is the daughter of Abuelita and her husband, Reggie's aunt (and therefore the sister of one of his parents) and the mother of Soos Ramirez. Little is known about Soos' mother, aside from her Latina heritage.
An unnamed Li'l Gideon fan is mentioned by the child psychic on his date with Mabel Pines. He apparently shooed her away, telling her to "autograph her own headshot."
The unnamed magician is a man who once went on a date with Melody, an experience she found unpleasant.
Wax John Wilkes Booth
Wax John Wilkes' Booth is one of several Wax figures from the episode "Headhunters." He is a wax figure of an infamous actor and the assassin of President Abraham Lincoln. When wax Abraham Lincoln is found melted, Stan accuses Wax John Wilkes Booth of doing it.
Wendy's cousin owns a logging camp in upstate Oregon.
Wendy's ex-boyfriends are numerous males formerly involved in romantic relationships with Wendy, whom she mentions when talking to Mabel about breakups. These individuals include:
Unnamed guy with tattoos
Wendy is uncertain as to whether or not she broke up with Mark, which explains his constant calls to her.