Rosa Valdez (also known as Aunt Rosa) is Leo Valdez's aunt.
When Leo's mother died in a mysterious factory accident, but Leo survived, his Aunt Rosa blamed him for the fire that killed her sister. She calls him a diablo - a devil in Spanish, and turned his whole family against him. She refused to take him in so the social services sent him to a foster home, which he promptly ran away from and for that reason Leo hates her.
Aunt Rosa is mentioned by Leo when he thinks of the time when his mother died in a factory fire, and his Aunt would not take him in because she blamed him for the fire.
Leo sees Nemesis, the goddess of revenge, as Aunt Rosa, a person he wants revenge at who judged him unfairly.
When Leo sees Nemesis in the form of his Aunt Rosa, he says that she has the same broad nose with a mole on one side, along with the same sour mouth and hard eyes.
Leo realizes that it is not Aunt Rosa, as Aunt Rosa used to wear floral dressing gowns, not red leather jackets like Nemesis.