In the movie "Cowboys & Aliens," Ella Swenson is a mysterious woman played by Olivia Wilde. She is introduced early in the film as a lone traveler who joins forces with Jake Lonergan (played by Daniel Craig) and the other townspeople of Absolution in their fight against the invading aliens.

Ella is initially a bit of an enigma, with her motivations and true nature left unclear. However, it is eventually revealed that she is actually an alien-human hybrid who was created by the aliens as part of their experimentation on humans. She has been sent to Earth to help stop the invasion and to prevent the aliens from taking over the planet.

Throughout the film, Ella serves as a valuable member of the group and becomes an important ally to Jake. She is intelligent, resourceful, and a skilled fighter. She also shares a romantic connection with Jake, although their relationship is complicated by her true identity.

Ella's character adds an element of mystery and intrigue to the film, and Olivia Wilde delivers a strong performance, conveying the character's inner conflict and vulnerability. Her ultimate sacrifice in the final battle against the aliens is a poignant moment in the film.