Stephanie Brown Headcanon
In the time between Cass becoming Bagirl during No Man's Land and actually meeting her Steph was jealous that Cass had been offered Batgirl.
While she never considered becoming Batgirl during her tenure as Spoiler the fact it was seemingly available and yet never offered to her while this new girl just gets it handed to her as soon as she showed up irritated Steph.
The fact Steph is more similar to Babs than Cass is, teen girl inspired by Batman to put on a costume and go help people + has a thing for Robin vs trained assassin looking for attonement, didn't help. Though Cass, Stephand Babs are all similar in various ways.
Cass is the only person she has admitted this to and it played a part in why Cass gave the mantle to her. (Side-Headcanon it was Cass's idea not Bruce's because it being his idea is dumb. She suggested it after he asked her to go to Hong Kong and he agreed)