Mon-El won't be the only major character returning for Supergirl's 100th episode.
TVLine reports Odette Annable and Sam Witwer will reprise their roles as Samantha Arias, aka Reign, and Ben Lockwood, aka Agent Liberty, respectively. Details about their roles are being kept under wraps, so it is unclear if they will come back as villains or their mild-mannered counterparts.
The 100th episode, titled "It's Super Life," will see Mxyzptlk offer the opportunity of a lifetime: the chance to go back in time and confess her secret identity to Lena Luthor before she finds out from Lex. This will trigger a series of events that will highlight major moments from the series so far and make Kara weigh her chance at redemption against the greater good. It seems this will also enable Sam and Ben to return, despite the latter's prison sentence.
According the official episode synopsis,
THOMAS LENNON (“RENO 911”) GUEST STARS IN SUPERGIRL’S 100TH EPISODE – Mxyzptlk (guest star Thomas Lennon) returns with a proposition for Kara (Melissa Benoist) – what if she could go back in time and tell Lena (Katie McGrath) her secret before Lex did, would things turn out different? Looking back over key moments throughout the series, Kara must decide if she wants to change history to become friends with Lena again. Familiar faces return in this special episode. Jesse Warn directed the episode with story by Robert Rovner & Jessica Queller and teleplay by Derek Simon & Nicki Holcomb (#513). Original airdate 2/23/2020.
Airing Sundays at 9 p.m. ET/PT on The CW, Supergirl stars Melissa Benoist as Kara Danvers, David Harewood as Martian Manhunter and Chyler Leigh as Alex Danvers.