Kitten was a teenage girl living on the colony of Kyaral when Daram attacked the planet with Sublight missiles. Her father, a soldier, was killed in the attack. She took care of a group of orphans. Initially, Kitten was distrustful of Cosmo when he arrived, first suspecting him of being Buff Clan, and then blaming him for prolonging the fighting. However, they soon fell for each other, and Kitten was instrumental in persuading Parkinson to allow the colonists use of the Stagra armoury.
Soon afterwards, while Cosmo was fighting the Buff Clan in space, Kitten was killed by Daram when his escape pod landed on Kyaral. Her death deeply traumatised Cosmo. Their spirits were eventually reunited when he was killed, her kiss helping his spiritual form awaken.
Kitten was given a much more violent death in Be Invoked, where she was decapitated in full view of Cosmo. Personally I've always chosen to interpret this as his nightmare version of events, though that wasn't Tomino's intention. Her name is a little unusual, even by this series' standards - characters tend to call her Kitten as if it's her given name, or even Kitty Kitten. It's possible Kitty is her family name and her parents were evil, or that it's just a nickname and her family name is unknown.