Disney-Wiki describes Thomas like this:
Thomas O'Malley comes off as a smooth-talking, streetwise cat who relishes his life of freedom out in the open with no rules and no responsibilities. All of this changes when he meets Duchess for the first time. He falls in love with Duchess when he first meets her, and he quickly starts flirting with and serenading her. Although he was obviously a little turned off by the fact that Duchess has kittens, but he and the kittens quickly grow on each other and he becomes like a father to them. Also, when faced with an adversity, O'Malley will indeed rise to the occasion to protect the ones that he cares for and loves.
What a guy!