Jeri Ryan as 7 of 9: Main character in VOY and a fan favorite. Was mainly brought back because of her past as a Borg drone like Picard/Locutus. But is written to be a completely different character from what we saw in VOY, and there are zero references to her time on Voyager or anything else having to do with the crew or that show. This character (former Borg drone now in some space vigilante group with a hint of lesbianism) could easily have been played by anyone, and could easily have been a new character since she really has nothing at all in common with the 7 of 9 character from VOY other than the actress who plays her.
she’s a “different character” because in Picard it’s been 20 years since Voyager got home. That’s a lot of time for her to actually grow and change. She wasn’t going to stay that stiff and socially awkward ex-drone that wore tight catsuits forever. Especially surrounded by so many humans and especially because we don’t know what all her experiences have been.