Well, in my opinion Osiris is awesome! Her heal isn't that strong but it offers +15 burst gauge for your entire party. Furthermore it has a short CD (5 Turns).
Her dizziness skill is also nice in Accessory Quest it helps me a lot sometimes.
Later with Pluto + Satan awakened and Osiris ofc you will have a good sinergy and a fast burst build.
Therefore I would pick Satan or Osiris now but... I am little bit too late to recommend you something.
However you have two core light Himes now. Therefore I would try to complete it and grab another good SSR light Hime with the next miracle ticket.