Kes

What do you mean you can't euthanize my boyfriend?

Kes was an Ocampan female, who spent three years on the USS Voyager. She did little of note during her time on Voyager, asides from nearly killing Tuvok on more than one occasion, eating some beetles, and being the girlfriend of Neelix for a short while before dumping him.

Shortly after being in telepathic contact with Species 8472, the occasional psychic powers Kes displayed suddenly and inexplicably increased to a massive degree, forcing her to leave the ship. Afterwards, she "evolved" into a non-corporeal life form (although as usual, the process had little resemblance to real-world evolution), and finally did something useful for Voyager, propelling the ship 10,000 lights years closer to Earth, conveniently bypassing Borg space.

Some years later, she again went another inexplicable change, "de-evolving" back into an Ocampan with powerful psychic abilities and anger management issues. Despite coming up with a bizarre plan to get herself killed in the past by handing Voyager over to the Vidiians, Janeway talked her out of it, and Kes took a shuttle back to the Ocampa homeworld, never to be seen again.