There could be parental or guardian - like characters. Maybe an older Necrofriggian or an alien from another species who takes on the role of protecting and guiding the babies. These characters would play important roles in the development of the story as they interact with the babies.
Well, the Necrofriggian Babies themselves are obviously main characters. They are the focus of the story, and their actions and experiences drive the plot.
In some cases, there might be human characters if it's a story that involves a crossover. Scientists or adventurers who stumble upon the Necrofriggian Babies and then become involved in their lives, either out of curiosity, a sense of duty, or a desire to study these unique creatures.
Another possible antagonist could be an alien scientist who was involved in the creation or discovery of the Necrofriggian babies. This scientist might have his own evil plans for the babies and sees Ben as an obstacle in his way. He could be using his advanced technology to try and capture the babies from Ben.
Another theme might be their growth and development. How they adapt to different situations, learn new things, and form relationships with other characters in the fanfiction can be a popular theme. This could involve their relationships with other alien species or even humans if it's in a crossover scenario.
In this fanfiction, it could be that the Necrofriggian babies are the result of some sort of experiment gone wrong. Ben stumbles upon them and realizes that he is the only one who can help. He has to figure out how to reverse the effects of the experiment while also dealing with the fact that the babies are in a very fragile state. He might enlist the help of his friends, but also face challenges from those who don't want the experiment to be reversed.
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.
Since it's a Harry Potter fanfiction, it could involve the main characters from the original series like Harry, Hermione, and Ron. Maybe they are the ones dealing with the situation of the babies being 'for keeps'.
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.
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.
Naruto is almost always a main character in 'naruto dice fanfiction' as the story is based on the Naruto universe. His friends like Sasuke and Sakura also often appear. Sasuke's cool and brooding nature can add an interesting dynamic when combined with Naruto's more upbeat and impulsive personality in stories related to the dice.