User blog comment:B214/General Discussion/@comment-104.7.80.57-20161109004004/@comment-10330822-20161117213020

Everything from the end of season 3 is non-canon, please don't forget that, so such a character development shouldn't be happening unless they choose to take the anime in a different direction, which I feel would be a mistake. Issei's overcoming Raynare's betrayal is a part of him finally confessing to Rias during his fight with Sairaorg in Volume 10, its a big, powerful point in the series. Issei proves he is the strongest young devil, and admits his love in one very powerful scene.

Issei does occassional refer to Rias as Rias or Rias Gremory when she isn't around, he doesn't call her President 100% of the time unless she is around, and even in the rare cases that he doesn't he will refer to her very impersonally when talking to others as "His King, Rias Gremory". Volume 10 is the first time he didn't do that, he simply called her Rias to her face.

Also when he is about to die in Volume 11, he simply smiles and says "I love you, Rias". Which is less about him calling her by her name, and more about the fact that, in his final moments, she is who he is thinking about.

Rias of course doesn't know this. So you can't really blame her for being insecure sometimes, because she doesn't feel all that special compared to the other girls. That is why she is so defensive of their relationship. However, by volume 19, she is seen to be more secure with their relationship, as such she isn't very upset when Issei takes Rossweisse out on date, despite Akeno becoming upset.

Ultimately, Rias either has or will wake up to the fact that Issei does treat her first. She was one he confessed to first, she is the first one he proposed to, and most likely, the first one he will marry and the one he gives his first time to.

It seems like the other thing Rias doesn't beat the other girls to is having his first kiss, which Irina took, and having his first born, which Akeno seems to take.