Oh, I read dark Ama :P My fault.
And Pluto is nice for a damage build with Satan Awakened *-*
Well, Crimson is right. I also have Hypnos in my sub and she is really nice with her ally regeneration. If Osiris is dead then I know that it won't be a big deal because of Hypnos but her CD is a bit long...
Furthermore she also offers light resistance for your entire team.
Just like Crimson said you are fine with Dark Amon because next year her -25 def down will be a C-frame and you will reach -50 def down with Dark Amon + Satan awakened or Samael.