The dragonborn himself is, of course, a main character in these fanfictions. Depending on the story, other DxD characters like Akeno Himejima could also be involved. She has her own powers and personality, and her interactions with the dragonborn can add a lot of depth to the story. Her seductive nature might play off against the dragonborn's more straightforward and perhaps stoic personality.
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.
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.
There could also be characters like Jon Snow. His noble nature and his own journey in Westeros could intersect with that of the dragonborn. They might fight side by side against common enemies, or they could have ideological differences that lead to conflict.
Alina is also frequently involved. Her connection to the Darkling in the original work makes her a natural addition in many fanfictions. Their complex relationship can be further explored or even rewritten in different ways in fanfiction.
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.
Obviously Naruto himself is a main character. He is the one with the pacemaker so he is central to the story. Sasuke might also be involved. He could be initially skeptical of Naruto's new situation with the pacemaker but then come around to help. Sakura too, as a medical ninja, she could play a crucial role in understanding and perhaps even trying to modify the pacemaker.
There can be original characters created specifically for the fanfiction. For instance, a detective who is obsessed with bringing down the Joker but slowly starts to lose his own sanity in the process. This character can provide a new perspective on the Joker's reign of terror.
Well, obviously Gaster is the central character as it's about his pregnancy. But often, other characters from the same fictional world might be involved. For instance, characters who are friends or colleagues of Gaster could be there to support or oppose the situation. It might also include characters who have some sort of special connection to Gaster's unborn child, like a potential mentor or an enemy who wants to harm the baby.