Blargetha is the younger sister of Slime Princess. She first appears in "Love Games," in which she plans to take over the Slime Kingdom and turn it into a military force, to take over all of Ooo. In order to get control of the kingdom from her sister, she has to get married, but by Slime Person standards, she is so attractive that no man can work up the nerve to approach her; therefore, she makes a fake husband named Guillermo, and was about to take over the kingdom until Slime Princess convinced Finn to pretend to be her husband. They eventually find out the truth about Guillermo, thus disqualifying her from the challenge for the kingdom's rule.
She looks almost identical to Slime Princess in coloring. Instead of a crown, she wears a band around her head with a green oval-shaped gem hanging down.
Her fake husband was made of green gelatin and olives. "He" had a long handlebar mustache and a feathered hat.
- She appears in "Royal Ruckus" in the second dungeon as a boss who is driving a tank.
- Her name is probably a distortion of "Lagertha", the name of a legendary shield-maiden from the Norse mythology. Her intentions to raise an army and fight reinforce this possibility.