Althea is a fellow alumni of Boulder High School, alongside Mindy and Glenda Faye, who they meet up with again at their reunion in the Season 4 episode Long Before We Met. She was played by Marilyn Kagan.
About Althea[]
Althea arrives at the reunion around 9 months pregnant by her appearance, and vastly enthusiastic. She is elated to see both Mindy and Glenda and treats them both as old friends bouncing around first with them and then Mork, before realizing she doesn't know who he is. Mork introduces himself to her as Mindy's better half, but when Mindy has to make the return introduction she can't, and is forced to address her pregnancy instead.
Fortunately for Mindy, Althea is distracted by Dickie Nimitz who she used to have a lot of fun with in the library, probably indicating enough about Althea's personality as to why she is in the family way.
After she's gone a mystified Mindy turns to Glenda to ask who that was, Glenda reminding her that that was Althea...adding that she used to play in the marching band when that doesn't ring a bell with Mindy, only for that not to ring a bell either.
The intimation clear that being so popular at high school, Mindy perhaps wasn't as aware of others, or at least as gregarious as Glenda was.