Issei is often a central character in these fanfictions. Since he is the main protagonist in the original series, his relationships and potential betrayals are explored. For example, a situation where his friends or allies in the Occult Research Club turn against him for some reason, like jealousy over his growing power or a misunderstanding about his actions.
Rias could also be involved. As a powerful devil and Issei's love interest in the original, a fanfiction might explore a scenario where she is betrayed by someone in her peerage. This could be due to internal power struggles within her group or someone being influenced by an external enemy. Her reaction to such a betrayal would be interesting to explore as she has to maintain her leadership and protect her territory.
Akeno is another character that could be part of these stories. Her loyalty to Rias could be tested in a fanfiction where there is a betrayal within the group. Maybe she is wrongly accused of being the one who betrays, or she has to choose between her loyalty to Rias and some other factor that could lead to a perceived betrayal. This would add a lot of drama and tension to the story.
Obviously, Escanor is a main character. He is often the center of the story due to his powerful presence. In addition, characters from the original Highschool DxD like Issei might also be involved. Issei could be a friend or a rival to Escanor in the fanfiction, adding an interesting dynamic as Issei is known for his own growth and development in the original series.
Rias is also a common character in these types of fanfictions. Her status and powers make for some really engaging body swap scenarios. For example, if she swaps with a less powerful character, it can show a different side of her as she has to adapt to the limitations of the new body.
Well, considering the nature of 'highschool dxd', characters like Xenovia could be involved in the 'ultimate hypnotism fanfiction'. Her strong will and combat skills might make her a target for hypnotism to be subdued or she could be the one trying to resist hypnotic control. Also, Asia, with her kind and gentle nature, could be a character that gets caught up in the hypnotism plot, either being hypnotized to do things against her will or being the one to break free from it and help others.
Issei is often involved as he is one of the central characters in the Highschool DxD universe. His character can be greatly influenced by hypnosis, either being the one using it or being affected by it.
Issei is often involved. Since he has a connection to the Gremory family through Rias, his actions and decisions can sometimes be a part of the Gremory - bashing scenarios. Another could be Sona, as her family has a relationship with the Gremorys and she might be in a position where she has to take a stance against them in some fan - created situations.
Rias Gremory could also be a main character. Her relationship with Issei and her role as a devil leader means she can play a crucial part in fanfictions about the One Winged Angel. She might be involved in helping Issei master the form or dealing with the consequences of its appearance.
Rias Gremory is often a popular character in such fanfics. Given her central role in the original Highschool DxD series, her relationship with Issei could be explored in the context of a pregnancy. Her status as a devil and the leader of her peerage would add a lot of complexity to the story if she were to become pregnant.
Rias is another common character paired with Gasper. Since Rias is the leader of the peerage that Gasper is a part of, fanfiction might explore their relationship in terms of leadership and following. For example, how Rias helps Gasper to overcome his fears and grow as a member of the group.
Natsu, being a fiery and passionate character, would likely have a rather intense interaction with the DxD Highschool characters. He might be initially distrustful due to his recent betrayal. But some of the more friendly characters in DxD, like Issei's group, could try to befriend him. Natsu would probably show off his powers a bit, which would either impress or intimidate the others. And there could be some clashes in personalities, for example, Natsu's straightforwardness against some of the more devious DxD characters.
In some fanfictions, the OC is betrayed by the entire group they thought they belonged to. This could be a club or a team within the Highschool DxD universe. The group might turn on the OC because they see the OC as a threat to their status quo. Maybe the OC has powers that the group fears will disrupt their power balance. This kind of betrayal can lead to the OC having to find new allies or even go it alone, which can be a great opportunity for character development.