Malaria is a one-shot love interest for Grim from The Grim Adventures of Billy & Mandy episode, Grim In Love.
Grim first met her at the beach and he thinks that she's beautiful. She and Grim hung out in graveyards. The two began a relationship, but their relationship didn't go well.
After Grim took her to his favorite bar and introduced her to his netherworld friends, she discovers that Grim is actually the Grim Reaper and her friends were real monsters. She panicked, runs away and she is never seen again.