Closure
After twelve appearances in the first season, Ellie disappeared with no explanation offered the audience. Behind the scenes, Donahue decided she and Griffith lacked the chemistry to make their on-screen romantic relationship credible and she decided to leave. Griffith later admitted that it was his own fault because he had a hard time showing affection on-screen, and as a result, the relationship didn't appear real or believable.
Read more about this topic: Elinor "Ellie" Walker