Joining Dan Stevens, Aubrey Plaza, Jean Smart and Jeremie Harris in FX's Legion will be Amber Midthunder (Banshee, Spare Parts, Sunshine Cleaning) as a character named Kerry. Deadline describes her as “a woman of action with a childlike sense of wonder. A true savant, Kerry has lived a sheltered, but never dull life.”
It's possible this Kerry will turn out to be an established Marvel Comics character, but for the time being we'll have to assume she's been created for the show. It's not even clear whether Stevens' take on David Haller will be tied to the X-Men universe or not - let alone Charles Xavier's son as he is in the comics.
Legion is set to kick off production early next month, and is expected to premiere on FX sometime during the fall.