According to the report, Ballerina will center on a young woman—described as an assassin—who's out for revenge against those responsible for murdering her family. Unity Phelan, notably, appears in the Parabellum credits as "ballerina," though the Deadline report makes clear that it's not yet known whether that's the same character.
However, it has been confirmed that whoever this character turns out to be was seen briefly in the most recent Wick entry. Possible involvement from Keanu Reeves has been neither confirmed nor denied.
A Dance Spirit report on Phelan’s involvement circa May of this year described her character as "a ballerina who also happens to be training to become an elite assassin."
Boasting a script from Shay Hatten (Parabellum), the spinoff is said to be on the fast track toward production just as Lionsgate is also moving quickly on a fourth Wick movie. The Ballerina project has actually been on the table for several years now, with reports on its development dating as far back as 2017.