“I have a thirst for vengeance, Grim. The kind of thirst that you get when you’ve been in the sun for a really long time and for some reason you keep eating ice cream“
- Velma in Wrath of the Spider Queen
According to Wikipedi.com:
Velma was once Grim’s best friend during their days at Doomsday Junior High together, but their friendship turned bad when Velma thought that Grim cheated to be the Grim Reaper (in reality, Grim had tried to help Velma win.) She was the main villain in the TV Special ‘The Grim Adventures of Billy and Mandy: Wrath of the Spider Queen‘. She was Jeff‘s bride to be, but Billy told Jeff she wasn’t good enough for him and Jeff canceled the wedding.
She was originally going to be the Grim Reaper but she thought he was cheating so he could win which destroyed their friendship. When Mandy learned the truth, she showed Velma, and she started to cry. Grim revealed that all the votes were for her because she would’ve made the best reaper. Velma renews her friendship with him when he told her the truth of what really happened. Grim found out that the teenage Boogey Man was cheating by putting more votes for himself so he can embarrass Velma after she lost.
Enraged, he unleashed a terrible display of his super natural powers–stemming from all the pent-up inner rage that she had once assured him that he had inside himself all along–and banished Boogey from trying to embarrass Velma which lead to everyone who hadn’t already voted to vote for Grim instead.
When he asked her what she was going to do next she said she has nothing else to do but rule the world like her dad (Skippy) wanted her to (her mom thought she had the power to rule two planets).