I wasn't expecting this choice, Ragna seemed the most obvious one, but this is okay too
Is because the artist is making a reference to the Seiyuu connection: Mai and Ruby (Hayami Saori), Nora and Elphelt (Suzuki Aya), Ramlethal and Penny (Han Megumi).
Someone datamined the game and it's confirmed Mai will be in the game as a DLC character.
and now everthing comes in full circle now with the addition of Mai in the game , and more hilarious as well to had a triple hayami saori party (Ruby , Mai and Orie)