A character with this trait is dishonest. Perhaps they're not frequently dishonest (e.g., like a con artist or a pathological liar would be), but a dishonest act in the story defines their character and/or is a prominent plot point.

Dishonesty is to act without honesty. It is used to describe a lack of probity, cheating, lying or being deliberately deceptive or a lack in integrity, knavishness, perfidiosity, corruption or treacherousness.