Cate Whitlen is Ree's futuristic vampire hunter. She has a wild dye job (which changes often; it's been purple, blue, and zebra at different times) and cybernetic implants. The implants increase her strength and stamina, and let her release electricity through her metal fingernails. The hair is just for the hell of it.
Cate carries loose wires the pockets of her beloved trenchcoat. They're for recharging her internal battery. Her implant system is supposed to charge itself off her movements, but she drains it too quickly and thus requires additional electricity to keep going. She has an electrode in her brain that makes her feel wonderful when it's electrically stimulated (and for some time afterward), a sensation akin to an orgasm. Her low battery attests to her overuse of this electrode... I'll let you piece the information together.
She is grey-eyed and left-handed.