Keiko's Links
He's her smart aleck, aloof kitty, the tsukkomi to her boke. She adores Taiga as only an airhead can adore the cat that takes more care of her than she does of him.
Keiko is his owner, and he lets her think she's the one in charge of the relationship. Even though he's supposed to be a cat, he's constantly having to remind her of things she's forgotten and help her keep things together.