Grandma Estevez is the grandmother of Sheen Estevez. She is never seen, but she is mentioned a few times in Jimmy Neutron and Planet Sheen. In a few episodes, Sheen would write to her about what has happened that day. However, she never gets any letters.
Skunky Beaumont is a student at Bluffington Elementary School who is often mentioned, but never seen in the original Doug series. His voice is heard offscreen in the episode "Doug Didn't Do It", and he is also mentioned in the episodes "Doug's Cool Shoes", "Doug Takes a Hike", "Doug on First", "Doug's Nightmare on Jumbo Street", and "Doug's Brainy Buddy". It should be noted, however, that Skunky eventually did appear as a recurring character in the series' continuation under Disney Television Animation, in which he was voiced by Doug Preis.
Grandpa Fairywinkle is Nana Cadabra's husband. He is the father of Big Daddy, the grandfather of Wanda and Blonda, and the great-grandfather of Poof. He is likely Italian, like his sons. He and Nana Cadabra are the first known members of the Fairywinkle family.
Mrs. Fairywinkle is the mother of Wanda and Blonda Fairywinkle, the wife of Big Daddy, and the maternal grandmother of Poof. She has never made an appearance in the show, nor has her name been revealed. She was first mentioned in "Blondas Have More Fun!" by Wanda, who said to Blonda, "Mom always liked you best!" Also, in A Fairly Odd Christmas, Cosmo stated that "I haven't been that scared since Wanda's mother moved in with us."
Mrs. Tang is Trixie Tang's mother, mentioned in "A Wish Too Far!" when she tells her friends that, "My mom says we can go to the pizzeria, and then buy it!" Despite this, she has never been seen at Trixie's house, nor has she appeared when Mr. Tang was present in gatherings with other parents, leading some fans to speculate that something may have happened to her, possibly divorced from Mr. Tang.
Fuzzy Slippers is Gerald's mysterious informant, and source of many local myths and urban legends. He is never seen and his true identity is never revealed, if he exists at all.
Fuzzy Slippers was first mentioned in "Door #16" when Gerald claims that Fuzzy Slippers wears bowler derby hats, and purchases his from Toller's Bowlers. He is then mentioned in "Cool Jerk" when Gerald claims that Fuzzy Slippers had informed him that Frankie G. was "bad news." And in Hey Arnold!: The Movie, Fuzzy Slippers gives Gerald the address to Bridget's base.
Mr. Smith is one of the boarders at the Sunset Arms Boarding House during the show's first season. He avoids virtually all contact with the others, including Arnold, who has to collect the rent through a vault hidden in the floor (as seen in the episode "Gerald Comes Over"). He had lived in the boarding house for three years, but has never said a word to anybody. It is implied in "Door #16" that he is a government agent when Arnold and Gerald visit the place he works at. Mr. Smith is not even mentioned again in later seasons; according to Craig Bartlett, he moved out.
Peapod Kid's father
Despite being a "Mrs.", Mrs. Vitello's husband has neither been shown nor mentioned within the series run.
Aunt Jennifer is Freddie's aunt. She is never seen or heard, as she was only mentioned while Freddie argued with her over the phone in "iMeet Fred". She apparently is a Fred fan, as she was calling Freddie names over the phone because he had said things that made Fred stop making videos.
Amanda is a cousin of Freddie. She is mentioned in "iSaved Your Life" and described as "icky", (by Carly), and "disgusting", (by Freddie). Dan mentioned in his "iSaved Your Life Fun Facts" that Amanda would be seen in a future episode, but she ultimately never did appear.
Freddie's Uncle is mentioned in the episode "iHatch Chicks". Freddie says that his uncle is a carpenter and maybe he can help get the baby chick out of the wall.
Leonard Benson is the father of Freddie Benson and the ex-husband of Marissa Benson. In "iChristmas", it is mentioned that Marissa is single. It is possible he left because he couldn't stand her anymore (or vice versa).
Dr. Lopez is the McBride family's oft-mentioned therapist. She helps Howard and Harold McBride with any of their family troubles they're having, and also helps Clyde deal with his obsessive crush on Lori.
Gayle is Clyde's adoptive grandmother, and the mother of either Howard or Harold. Clyde sometimes mentions her, imparting words of wisdom he has learned from her. Her name is given in "Snow Way Down".
Simon Sharp is Sam Sharp's brother, currently only mentioned in Sam's bio in The Loud House graphic novels. Little is currently known about him other than that Sam believes her life is chaotic enough with just him, as opposed to ten siblings like Luna has.
Barnacle Boy's son
Billy Bob Star
Dirty Dan and Pinhead Larry
Dirty Dan and Pinhead Larry are a pair of criminals in Sandy's dream in the episode "Survival of the Idiots", supposedly on the run from the law due to tax evasion. Sandy references Dirty Dan as a criminal before SpongeBob and Patrick create a game where they would fight over the titles of Pinhead Larry and Dirty Dan. SpongeBob pretends to be him at first, but at the end of the fight, Patrick is titled as Dirty Dan. During this fight, Sandy confronts them while sleepwalking and asks who the real Dirty Dan is.
Grandpa Cheeks is the grandfather of Sandy Cheeks. At some point, he went into space and invented a surfing move called "The Tour to Saturn." Later, he taught his granddaughter Sandy how to do the move.
Great Grandpappy Krabs
Great Grandpappy Krabs is Eugene Krabs' great-grandfather and the grandfather of Victor Krabs and Aunt Sally and the father of Grandpa Redbeard. is only mentioned in "The Curse of the Hex" when he mentions that there are no such things as curses. Mr. Krabs claims that Mr. Krabs' great-grandfather invented a money security system in "Pest of the West".
Larry SquarePants is SpongeBob SquarePants' cousin, mentioned in the video game SpongeBob SquarePants: Operation Krabby Patty. When SpongeBob wakes up and tries to find Gary, Plankton uses a tape recorder to tell SpongeBob he has stolen his pet. Plankton, however, mistakenly calls him "Larry" at first, causing SpongeBob to think that Plankton has stolen his cousin Larry.
Mario is the owner of Pizza Castle, mentioned in the episodes "The Bully" and "Suction Cup Symphony". He is first mentioned when Patrick asks for him when he accidentally calls Mrs. Puff's Boating School, mistaking the school for Pizza Castle. Later in "Suction Cup Symphony", SpongeBob hears Squidward's clarinet playing and says "Sounds like Squidward ate at Mario's last night."
Maw Tucket is the grandmother of Patrick, Sam, and Gary. She is first seen in "Rule of Dumb", on the Star/snail family tree. And in "InSPONGEiac", an image of her legs appears on a projector screen as Patrick explains that she had toe barnacles and invented a cure that Patrick later revealed to be coffee.
Prince Callous was the great-great uncle of Patrick, Gary, and Sam, seen on the family tree in "Rule of Dumb". He seems to be some kind of dolphin. He is light turquoise. His scleras were dark green with black pupils. He wore a light blue shirt, white belt, gold belt buckle, dark blue pants, and a round, blue hat
Princess Tulsa was the great-grandmother of Patrick, Sam, and Gary and the grandmother of Herb and Sluggo, seen only on the family tree in "Rule of Dumb". She was lavender with white hair and wore a brown hat and brown ragged clothing. She was one of two children, and the only daughter of King Amoeba and Queen Mildew.
Sluggo Star is the father of Gary and the brother of Herb Star, who is seen on the Starfish Minister's poster in the episode "Rule of Dumb." He is a small light purple slug with a light green underside and yellow-and-black eyes. At the end of the episode, the elder starfish finds a coffee stain revealing Gary to be Sluggo's son, and the true and rightful king of Bikini Bottom.
Yorick is Princess Tulsa's husband and Maw Tucket's father. He has only appeared on Patrick and Gary's family tree in the episode "Rule of Dumb". He is a light orange fish. He has light green eyes with black pupils and red lips and he has a big head. He wears a light green and orange jester's outfit.