User blog comment:B214/Question and Answers 3/@comment-4547267-20140807065153/@comment-24098614-20140808012439

The honorific that Rias uses means how intimate their relationship has become because Japanese when they call you by their first name without honorifics means that you are lovers at a intimate level or a part of the immediately family meaning that Rias is closer to Issei than the rest of the girls because until the girls drop the honorific of his first name it means that their feelings towards him hasn't advanced to the intimate level like Rias.