Specifics about Purnell's role were not revealed, but according to sources the character will be "upbeat and uncannily direct with an all-American can-do spirit." Her casting follows that of character actor Walton Goggins, the first actor to be announced for the show, who will reportedly play either a ghoul, or a guy named Ghoul. (I suspect it's the former.)
I'm not familiar with Purnell's work, but a quick trip to IMDB reveals a range of roles in television and movies stretching back a decade, including—this is the one most immediately relevant to our interests—as the voice of Jinx in the League of Legends animated series Arcane.
Purnell also stars in the acclaimed Showtime series Yellowjackets, which was recently renewed for a second season, and the animated Star Trek: Prodigy series on Nickelodeon, and had a leading role in Zack Snyder's 2021 zombie flick Army of the Dead.
Production on the Fallout television series is expected to begin sometime this year, but a release date hasn't been announced.