Kit

Kit was the Halliwell's familiar.

Something Wicca This Way Comes
When Serena Fredrick prepares for her spell, she feeds her cat. The cat runs off when the serial killer enters and kills Serena. When Andy investigates the crime scene, Darryl mentions that the cat has been "clawing the crap" out of all the other officers. Andy however doesn't get scratched. After Phoebe's first premonition she crashes her bike saving two roller-bladers. The cat meows, watching from afar. When Phoebe and Prue get back to the manor after discovering their powers, Prue jokes about Piper turning into a cat and brings the cat in. The morning after the Charmed Ones vanquished Jeremy, Phoebe carries Kit outside when she hears Prue talking to Andy. Kit meows at Andy.

I've Got You Under My Skin
Kit jumps up onto the kitchen counter and meows in front of the TV, playing a documentary on Mary Estes.

Biography
At some point, Kit the cat became the familiar of Serena Fredrick, a Witch who was murdered by her warlock boyfriend, Jeremy Burns. Even before Serena was killed, Kit would keep appearing at the front door of the Halliwell Manor, the home of her later owners. When Jeremy came after Piper Halliwell, Kit managed to get inside the Manor by climbing through an open window. She stayed with them for three years. She disappeared shortly after Prue's death and after Paige was revealed as a Charmed One. Although the sisters did not know why, it was because her work as their familiar was done.

In the season 5 episode, Cat House, Kit returns as a human under the identity of Katrina. She was hunted by a warlock, portrayed by Zachary Quinto, who attempted to kill her, but she escaped. While traveling through time due to Piper's spell, he continued to try to murder Kit both in feline and human form. Had he been successful, he would have erased much of the good the Charmed Ones had done. Each of of his attempts to murder Kit were stopped by Phoebe and Paige, which led to his vanquish. It later emerged that she'd been rewarded for helping the Charmed Ones by becoming a human--something that is only granted to a few special familiars. She is now training the next generation of familiar.

Special Abilities
Kit/Katrina can communicate with other cats in both forms, superhuman agility, has enhanced senses, and has the ability to sense evil (especially warlocks).

Trivia

 * The original cat who played Kit in seasons 1 - 3 died shortly after the third season. This is why Kit never appeared in seasons 4 - 8. The cat playing in Cat House was another cat, the episode needed to explain why Kit left.
 * Kit was originally the familiar of the witch who was killed in Something Wicca This Way Comes.