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.
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'.
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.
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.
Naruto himself is often a central character in 'naruto destruction fanfiction'. Since he is the main protagonist of the original series, fanfiction writers like to put him in situations where he has to deal with destruction. Sasuke is also frequently involved as his rival, and their relationship can be explored in the context of destruction, like if they have to work together to stop a greater threat of destruction.
Obi - Wan Kenobi is frequently in Anakin Snaps fanfiction. As Anakin's master, he has a big role to play. Their relationship is often explored, and Obi - Wan's reactions to Anakin's snapping are crucial. Sometimes he tries to bring Anakin back from the brink, while other times he might be the cause, albeit unintentionally, of Anakin's breakdown.
Well, Naruto is obviously a main character in most of these. But there could also be Sasuke. He might be either an ally or an adversary in relation to the dagger. For example, if the dagger has a power that can either be used for good or evil, Sasuke could be tempted by its darker potential.
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.
Obviously, the Reaper is a main character. Ruby Rose is also often involved as she is one of the central characters in RWBY and her connection to the concept of death (which the Reaper represents in a way) makes for interesting interactions. Weiss Schnee can also be seen in some fanfictions, as her family's past and her own story can cross paths with the Reaper's storyline in creative ways.
Some fanfictions might also include characters like Kakashi in important roles. As Naruto's former teacher, he could be a guiding figure or someone who has to adapt to Naruto's new level of power. He might also be involved in trying to keep Naruto's power in check if it starts to have negative effects on the ninja world.