High School DxD and its characters belong to Ichiei Ishibumi and Fujimi Shobo.

Enjoy

*MAJOR SPOILERS AHEAD*

In the newest season, Rias begins treating Issei more coldly; partially due to Issei continuously calling her Buchou (in the English dub, he begins calling her Prez), apparently offending her. Several time throughout the season, Rias even asks Issei who or what she is to him, but everytime Issei gives her an answer (while calling her Buchou/Prez), she gets offended and snaps back at him.

In the first half of the season, the second-years of Kuoh Academy go on a field trip to Kyoto. Issei is soon attacked by the Youkai Kunou, who accuses Issei of kidnapping her mother. After clearing the misunderstanding, they are attacked by the true culprits: Cao Cao and the Hero Faction of the Khaos Brigade. Issei manages to achieve the Illegal Move Triania and change the Scale Mail armor through Promotion. Despite this, he gets overwhelmed by the Hero Faction before being saved by Sun Wukong.

In the second half of the season, Sairaorg Bael challenges Rias to a Rating Game (or Issei to a one-on-one duel), desiring to fight Red Dragon Emperor. After the battle in which Rias and/or Issei is victorious, Rias once again asks Issei who or what she means to him. But after Issei once again call her Buchou/Prez, she once again snaps at him. Fed up with Rias' strange behavior, Issei snaps back at her by calling her a "fucking devil", only to have Rias slap him in face and storm off, leaving Issei behind to get verbally abused by his friends as well. Crushed by their cruel words, Issei runs away and once again asks himself if Rias (or the rest of her peerage for that matter) actually like him at all. It is then that a familiar voice from his past whispers, "Will you die for me?"

Meanwhile, Rias and her peerage eventually figure out that Issei couldn't admit his feelings to Rias because his previous girlfriend he admitted his feelings to, Yuuma Amano, was actually the Fallen Angel Raynare who killed him for his Sacred Gear. Shocked and guilt-ridded by what they've done, they try to find and apologize to him for their behavior. But by the time they find him, they are too late; Issei had committed suicide by removing his Evil Pieces used to resurrect him. The next day, a funeral service is held for Issei with many people attending: including Rias and her peerage, Issei's grief-stricken parents, the now Perverted-Duo, and even the Kendo Club who hated Issei for his perversion were so shocked by his death that they attended as well. When it is her turn to speak, Rias explains to everybody that Issei was killed while trying her and the other girls from being kidnapped and raped by gang members. (It was a lie, but it was made to teach everybody that was Issei was more than just a pervert.) After the funeral ended and everybody else left, Rias stayed behind to give a long apology to Issei for how she treated him without understanding his problems. After her speech, she heads home as well. And the last thing we see before the season ends is red light emerging from within the grass above Issei buried coffin...

To Be Continued...

If those who are wondering (and not liking this story), before joining Rias' peerage, Issei was asked by a girl named Yuuma Amano to be her boyfriend. However, she was Fallen Angel named Raynare who became obsessed Issei's Sacred Gear and, at the end of their date, killed him after he admitted his feelings for her. This would haunt Issei so much that he felt that if he admitted his feelings to Rias, or any other girl for that matter, they would just mock him just like Raynare did. And this curse would continue onto the 10th light novel.

Rias, however, while unaware of Issei's curse, was more concerned about trying to force him to admit his feelings to her (in her case, it's Issei calling her by her first name instead of Buchou) than her concern for Issei's problems. In the light novels, (before the Bael/Gremory Rating Game) he tells his problems to Asia, Akeno, and Koneko (but not Rias) and they help heal his heart, lifting his curse. Then, after the Rating Game, Issei finally Rias by her name and admits his love, thus officially becoming her boyfriend.

But do you see the problem? It was almost like Rias knew about Issei's love issue but didn't care and just wanted him to call her "Rias". So in this story, Rias' words are what finally caused Issei to succumb to his curse and die, and by the time she realizes the truth, Issei's gone...all because of her pride and selfishness.

FYI, the season finale was actually based on Pandemonium1995's "Nothing Lasts Forever".

Also, did you not see the "To Be Continued..." at the end? This is my idea that instead of ending the series on a sour note, the story would continue in "High School DxD: The Movie", a film sequel to Hero that adapts the 11th and 12th light novels.

So you see, Issei was cursed by Raynare and, unlike the light novels, Rias' attitude caused Issei to succumb to his curse and die. But then, in "The Movie", Issei manages to come back to life and with help of his friends (except Rias herself), overcomes his curse and admits his feelings to Rias.

I'm sure many of you will still not like the summary, and that's fine. But this is just MY prediction (even though it sadly didn't come true as of yesterday). And even YOU have to admit, Rias never realizing Issei's problem is pretty selfish.

Later