Donna's little sister
We see her in one of the first episodes of season 1 when Donna's throwing a party at her home. And that's the last we saw of her.
There may be 1 or 2 mentions of her, a couple of episodes after that, like when Bob and Midge are going out and she tells Donna not to make her little sister eat dirt. But then that was it. I don't recall any explanation for her disappearance.
Did I miss something? Do any of you have ideas or a reason for it?