It could be two best friends. They've been through a lot together and the forgotten promise between them creates a rift that they need to overcome. For example, they might have promised to always be honest with each other, but one of them broke it unknowingly, leading to misunderstandings.
A couple in a relationship could be the main characters. They made a promise early in their relationship, like always being there for each other's big moments. But as life gets in the way, one forgets, and this causes problems. The fanfiction could then focus on how they deal with this issue, whether they can rekindle their love and trust by remembering and keeping the promise.
Since it's a Naruto - related fanfiction, Naruto himself is most likely one of the main characters. There could also be his friends like Sakura and Kakashi. Maybe Sasuke would be involved too if the promise has something to do with their relationship.
Another main character could be the godfather's most trusted consigliere. This person would be involved in planning and strategizing to help the godfather keep his promise. And of course, there would likely be an antagonist, perhaps a rival gang leader who wants to stop the godfather from fulfilling his promise.
A mysterious figure from the captain's past could be a key character. Maybe this person was involved in the events that led to the promise in the first place. They could reappear and try to either help or hinder the captain in keeping the promise. Also, a young crew member who looks up to the captain could be important. Their naivete and admiration could add an interesting dynamic as they try to understand the weight of the captain's promise.
One possible main character could be a vampire hunter. This hunter might initially be after the forgotten overlord but then realizes that there are greater threats in the vampire world and teams up with the overlord. Another character could be a witch who has knowledge about the spell that made the overlord forgotten and helps him to break it.
The mother's new partner. He doesn't want the forgotten daughter to be part of the family as he fears she will disrupt the new family dynamic he has been trying to build. He might use various means to keep her away from the family.
Since we don't have much information about the 'The Promise' novel, the main characters could be a group of people. Maybe it's a family where one member makes a promise to the others, and they all play a part in either helping or hindering the fulfillment of that promise. It could also be two lovers where one makes a promise to the other, and their relationship is built around that promise.
The forgotten king himself would surely be one of the main characters. He could be depicted as a complex figure, with both good and bad qualities.
I don't know for sure as I haven't read it. But it could be the main RWBY characters like Ruby, Weiss, Blake, and Yang. They might be the ones who get involved in the mystery of the forgotten gods.
The main character is surely the forgotten son. But we can't rule out other important characters. Sakura could be in the story, maybe she discovers some clues about the forgotten son. And Sasuke might also be involved, as he has a long - standing relationship with Naruto. Their children could also interact with the forgotten son, adding more layers to the story. Additionally, there could be some new characters created specifically for this fanfiction, like a wise old hermit who guides the forgotten son on his path.
The main character is Percy. However, in the fanfiction, there could be a new character created specifically for the story, like a young apprentice of Percy who has a unique ability. There may be an old and wise god who has some secrets about Percy's past and is reluctant to share them at first. Also, there could be a group of demigods who are initially skeptical of Percy but then come to believe in him and join his cause as he tries to reclaim his position among the gods.