Katherine is a victim of circumstance and Crey Technologies. While she was attending University she volunteered for a Crey Research project. Crey at first were going go go ahead and do some normal tests. When they realized that she had no family to miss her and few close friends they took the opportunity to do some more dangerous experiments on her. Katherine was subjected to a special serum that was designed to confer feline traits on the user, to increase agility and the senses. Katherine did not react well to the serum, going out of control because of what they had done to her, severely wounding several of the scientists with her new feline claws. Crey therefore considered the experiment a failure, the scientists attributing the rage to the serum, and not the violation of trust.
Katherine adopted the name Mynx, in respect to the cat. She lived on the streets of Paragon for quite some time, adopting the ways of petty thievery to keep herself alive and fed. She was discovered by the Hero Synapse, who took her in and gave her training to grow accustomed to her enhanced feline senses and agility, as well as control of her animalistic instincts.
Mynx is arrogant and overconfident in both battle and in her daily life, which might not be her own fault: instead a natural confidence and aggression that is ingrained in felines. She is prone to going feral in combat, lashing out with her claws and angering easily. She has a bit of a problem with authority, another feline trait. Despite this she is deeply devoted to Synapse, whom she considers like a father: someone who saved her life and saved her from herself.
She has a deep problem with Crey Technologies: liable to fly out of line and attack before considering the consequences.
On Praetorian Earth her counterpart is named Bobcat.
The writers sometimes mistake her name as being Katherine Steffan, but the majority of the sources write her name as Katherine Stevens. This is likely writers error, but it might also be a reference to the fact that she was known by few: a nobody before the Crey experiment and might not remember much of her life beforehand.