Nuclea (Julie Lemieux) is a Cruella de Vil-esque female alien with wild Medusa-like hair that is fatally shocking to the touch. She is energy-based being whose power (and size) increase as she consumes more energy and she has the power to conjure lightning blasts of various intensities and lengths. She continually uses new methods to secure large sources of energy to absorb. She is basically a human lightning bolt of energy, but that, of course, is not enough power for her.
In the episode "Nuclea Infected", she tried to absorb the gravitational power of a Grey Hole, which is a smaller, more managible version of a Black Hole, in order to be able to convert solid matter into pure nuclear energy on the subatomic level, making her unstoppable. At the end of the episode, she is presumed dead after Betty tricked her into sucking up proto-stars as she makes no appearances after this episode.
Her name is a play on the word "nuclear".