@mangatron: When Mina, Iria and maybe Kyuuru make their anime appearance, I will pick again the debate
@SkyBlackSwordsman: There isn't much more to say, Risara does owe at least her gratitude to Ryousuke for what she is putting him through, but unlike Rias is most of the time asexually tsun- to him.
What Rias and Risara have in common are their supernatural nature, status, admiration, presence, appearance (save for the cup-size).