from wikipedia
While her first name is Kate, she chose to go by KT as her first
name, saying "[Kate] just makes me think of a buxom lass baking
bread for her man working in the fields. I have no problem with
that, but it's just not really how I pictured being a rock star."
Also, Tunstall spells her first name KT (as opposed to Katie) to
differentiate herself from fellow singer Katie Melua.