Jessie's Step Brother Eric

Eric accomplished a lot in just two episodes. He moved in with Jessie, stole her room, blackmailed Zack and Slater, took apart Belding’s car (and put it back together), aggressively hit on Lisa, and eventually went on a date with her...and then, despite being Jessie’s step brother, living in her house, going to Bayside and being romantically involved with Lisa, we never saw him again.

Slater's Sister JB

Another sibling who was gone in an instant. Slater’s little sister, JB, arrived at Bayside, Zack took an interest, Slater initially disapproved, but then everything ended well at the school’s sock hop, where Slater gave Zack and JB his blessing. Despite the possible romance, JB’s status as a student at Bayside and her being Slater’s sister, we never saw her again.