Cassidy, who previously played Laurel Lance on Arrow, will return as Black Siren.
Katie Cassidy will be returning as an Arrow series regular for Arrow Season 6, proving that it pays to act in a TV universe where the multiverse is a thing. (Just ask The Flash's Tom Cavanagh.) That way, if your character is killed off, you can come back as an other-world version of your character.
Given that Cassidy's former series regular character Laurel Lance/Black Canary was brutally killed in Season 4, Cassidy will be returning as Lance's Earth-2 doppelganger Black Siren. According to The Hollywood Reporter, Cassidy will be back for the final two episodes of Arrow Season 5 before returning full-time in Arrow Season 6.
In a statement Arrow co-showrunner Marc Guggenheim said:
One of the things that most excites us about Arrow is that we go where the story takes us. Last year, the story took us to the tragic death of Laurel Lance. This year, our midseason finale reintroduced us to Laurel’s doppelgänger, Black Siren.
We were so taken by Katie’s fearless interpretation of this character that we knew her story was not yet finished. We couldn’t be more thrilled to welcome Katie back to Arrow as Black Siren, though as her appearances on Arrow and Legends of Tomorrow this season show, she never really left the family.
This is an unexpected, interesting choice — though not necessarily a bad one. Though Arrow didn't always know what to do with Cassidy's Laurel in the first few seasons of the show, her Black Canary was one of the better parts of Seasons 3 and 4 and her untimely death felt like a manipulative, contrived way to bring her character's story to an end.
Though Cassidy's return will not mean Laurel's return, she has been a lot of fun to watch on-screen as Black Siren in a role that is arguably a better fit for her. (Cassidy is so good at playing the morally-ambiguous-to-villainous role — see also Gossip Girl and Supernatural.) Her Arrow Season 5 appearance earlier this season was a particular highlight.
The re-introduction of her Black Siren character made Oliver confront his feelings about Laurel's death, as well as forced him to see Black Siren in a new light. "We get to choose what we are. We get to choose who we are. So, tell me, who are you?" Oliver told Earth-2 Laurel, in a particularly thematically-resonant moment.
The Black Siren/Felicity relationship also seems to have a lot of narrative juice left in it, especially given the apparent blurring of Felicity's right-wrong morality line in Season 5 (she wrote, without judgment). Past that, what will Thea think of the return of her dead best friend's doppelganger? (She was at that centuries-long conference the last time Black Siren popped around.) And how will Dinah feel about meeting the doppelganger of her costume predecessor? Diggle no doubt still has lots of guilt around his self-appointed role in Laurel's death.
Basically, the full-time Black Siren could give our characters lots of talk and feel about in Arrow Season 6. Color us intrigued...