Katherine was first introduced in the episode, "The Earth Contagion", as the Doctors' current companion. Her character is protective, abrasive and inquiring. She also prefers to be referred to as "Kat" as she finds her birth name to be too posh.
In the next episode, "Slaves to the Machine", she is supposedly killed by a rogue slave from BOSS's chemical plant. However, at the end of the episode we see her stir, implying that she is still alive.