Characters are cool addition. It's +1 character that may or not may be for you, and it's +1 matchup to learn that might be in your favor or in theirs. You also get all the fluff with them, like the new interactions.
This could be really great for a subset of the population that might main the new character, but it's not a lot for those that have no interest in picking up a new character.
I think what the Versusia patch, and most recently the Strive patch, showed us is that what really gets good retention is balance changes or content.
Strive got their new ranked mode which is a huge overhaul, even if it really is just correcting the mistake that the tower was in the first place. But they also got balance changes that from what I gathered were mostly positive.
Season 2 has had very anemic patches so far.