Missy is the love interest of Andrew and Lars in the Netflix animated comedy Big Mouth.
Missy is a friendly, chipper - albeit hyperactive - young girl. She comes from a bohemian household, has a crush on Nathan Fillion and a habit of dry-humping her plush toys.
Andrew and Missy start dating while working on a group science project together. When Andrew tries to kiss her, Missy freaks out and dumps him on the spot.
Andrew and Missy don't interact much after that, but when Andrew finds out that Missy still has feelings for him he sets about trying to win her back. In the leadup to Valentine's Day Andrew tries affecting an aloof and confident approach, but to his chagrin Missy only gives him a card out of friendship (she gives all the boys she knows cards). Andrew then gets desperate and starts performing several frantic gestures in the hopes that Missy will notice him, but when it becomes clear he doesn't know her that well he becomes possessive and temperamental. Missy then tells Andrew she no longer likes him and he breaks down.
Missy and Lars were close friends. At some point after Valentine's Day, they became romantically involved and were inseparable for awhile. When Missy discovers her wild side, Lars goes from supportive to possessive. Missy then dumps him for Nick, which causes Lars to team up with Andrew and try to ruin their relationship out of spite.
Nick and Missy didn't have any prior interactions outside of a spin-the-bottle kiss. When they're cast alongside each other in a school play that involves kissing, both begin to develop feelings of lust for each other. After the play's conclusion, Nick and Missy kiss for real, which Andrew witnesses.
Nick and Missy then have to deal with Andrew and Lars trying to break them apart. In the end it turns out it was all for nothing, as Nick soon discovers that he and Missy don't connect on a deeper level. They split up and Missy is left broken-hearted.