The Hollywood trade states that Tsai will play the mutant Switch in the series. In comic books, Switch is Devon Alomar, a former Hellion with the power to affect and alter peoples' brainwaves.
Variety describes Legion's version of the character as "a young mutant whose secret ability serves as the key to executing David Haller’s plan."
Though this is Tsai's first American screen role, she previously appeared on the Japanese reality show Terrace House. Interestingly, Tsai is also a working artist who drew a variant cover for Marvel Comics' West Coast Avengers #1 in 2018, seen here.
Legion season 3 is expected to premiere later this year, though no specific date has been announced.