Like the rest of her siblings, Kathleen is a doctor, trained in psychiatry. We meet her for the first time in season 15, where Amelia describes her as Martha Stewart if Martha Stewart had the ability to diagnose you with a mental condition. Like her elder sisters, Kathleen is married with children, though it's not specified how many.
Kathleen is played by Amy Acker, another well-known TV actress. She's starred in Angel, Person of Interest, and The Gifted.