The new FX series "Legion" has finally found its main character in "Downton Abbey" alum, Dan Stevens, whose casting was announced on Thursday. Stevens will be taking on the role of David Haller, a man who has been in and out of psychiatric institutions for his schizophrenia.

"Legion" is a spin-off of Marvel's "X-Men," featuring Professor Charles Xavier's son with an Israeli Holocaust survivor, Gabrielle Haller. David has been struggling with hearing and seeing things, which led to his diagnosis as a schizophrenic, but he will find that it is possible he's seeing real events after a bizarre incident involving a fellow psychiatric patient who also happens to be the girl of his dreams.

"A haunted man, David is getting tired of finding his way back to sanity and is about to give up when he meets the girl of his dreams, a fellow patient," Deadline's Nellie Andreeva describes Stevens' character.

Fortunately, FX also revealed two other important characters in "Legion." First is Aubrey Plaza who will play David's incredibly optimistic friend, Lenny. Lenny has led a life of alcohol and drug abuse but she is sure that her life will turn around somehow any day now.

The second is Jean Smart who will wear the robe of Melanie, a therapist with unconventional methods. Melanie has a sharp mind and is demanding but she also has a nurturing side to her.

The three new cast members will join Rachel Keller who plays Syd, the girl of David's dreams. She is a street-smart female in her 20s who, much like Lenny, is quite optimistic and chooses to believe in happy-ever-afters.

Noah Hawley will write the pilot episode of "Legion" and co-produce the series with Lauren Shuler Donner, Bryan Singer and Simon Kinberg who all worked together in the 2014 film "X-Men: Days of Future Past" and Marvel's Jeph Loeb and Jim Chory who are both executive producers for the Netflix series "Daredevil."