User blog comment:B214/Volume 22 spoilers/@comment-50.139.232.230-20161022060406/@comment-30289101-20161022063334

I would day Ross was more of a contractor to Rias. She inititially joined simply because it was a good deal, not do to any significant feelings toward Rias. At this point, it's clear Ross is more emotionally attached to Issei than Rias. When combined with the fact that Rias feels that everyone after Issei was drawn to her through his influence, she probably feels that all of them, Ross included, belonged to him in the first place; that she was merely a placeholder reserving them for him. Rias also wants to start fresh; to see what she could accomplish with the original memebers before Issei came and had heavy influence on everyone.