Matthew Mara (aka "Rickety Cricket") - Wore giant leg braces and was (and still is) in love with Dee. They were in physical therapy together twice a week after school. She promised to kiss him if he ate horse dung. He did so, but she refused to kiss him because his breath smelled like shit. Dennis and Mac used to teabag Cricket extensively in high school (and they still do at every chance they get). ("The Gang Exploits A Miracle")
Stacy Corvelli - Slept with Charlie. Tricked him into thinking he was the father of Tommy - her hellion son. ("Charlie Wants an Abortion") According to Dennis, she "was a slut". In a deleted scene on the Season 5 DVD it is implied that Stacy is the only woman Charlie has ever had sex with and that he did not enjoy it.
Peter Schmidt / Schmitty - The original fourth member of the Gang. He used to be roommates with Charlie. The Gang got tired of Schmitty and resulted in Dennis kicking him out of a moving vehicle and telling him "You're out of the Gang!".
Psycho Pete - Leader (or the "psycho back") of the "Freight train"-gang, which also contained Mac, Charlie and Dooley. After school, Psycho Pete cut his family into a little pieces and ate them for Christmas dinner. ("The High School Reunion, Part 2: The Gang's Revenge")
Dr. Larry Meyers - Dee's high school drama teacher who inspired her love of acting but in fact, was just an enthusiastic alcoholic. ("The Gang Gets a New Member")
Artemis Dubois - she's familiar with Matthew Mara, and also she's aware about why Waitress and Dee broke up with Brad Fisher; on the other hand, Dennis and Waitress at first seemed to be unfamiliar with her.
Ricky Falcone - Mac's family robbed his house on Christmas of 1985. Little Mac got himself an "Omnibot"-toy there - later, he's always thanked Ricky at school for it (most likely, it was elementary school). ("A Very Sunny Christmas")