Melissa is Curtis’ alter ego when he’s swapped genders. Curtis kept her a secret, only going public when he wanted to join the female athletics team, where he fell in love with Emma, who only liked him as Melissa.
After Curtis accidentally impregnated Melissa (yes, he knocked himself up by masturbating), he sold his power to Seth. This caused Melissa (and the baby) to cease to exist.
Even if you don’t consider that to be Mel’s death, Curtis killed himself after becoming a zombie, so she’s very most sincerely dead.