Often, the main character is a human or some other non - dark elf race who gets captured. Then there are usually the dark elves themselves, with their leaders or powerful warriors being important characters. Maybe there's a dark elf princess or prince who has a role in the captive's fate.
Typically, the dark elves are shown as powerful. They have advanced magic skills that they use to capture and control the human. They may be seen as cruel, especially in the beginning when they first capture the human. But sometimes, they are also shown as having a complex moral code. So while they might be doing bad things like capturing the human, they might also have reasons like protecting their own kind or following an ancient prophecy.
One interesting plotline could be the protagonist's struggle to escape from the dark elves' lair while facing various traps and challenges they've set. Another might be the development of an unexpected alliance with a sympathetic dark elf who helps the captive navigate the complex social and political structure within the dark elf community.
Often, the main character is the human who gets captured. They can be an ordinary person from our world who is suddenly thrust into the Pokémon universe and the strange situation of being inside a Pokeball.
Often, a plotline in such fanfiction is the human being captured for a ritual. The dark elves might think the human has some special essence or power that they can use in their dark magic rituals. The human, of course, resists this. Another aspect could be the relationship that develops between the human and a particular dark elf. Maybe they start off as captor and captive but gradually develop a strange friendship or even a romantic relationship, which is a very common trope in this kind of fanfiction.
The hunter is obviously a main character. They are the ones who brave the dark and face the unknown. Another could be the various creatures of the dark that they encounter, which can be considered as important characters as they drive the action.
Typically, there is a 'Papa' character who is likely to be a central figure. There might also be a younger character with whom the Papa has some sort of relationship. But it really depends on the specific fanfiction as different writers may create different main characters.
Obviously, Bella is the main character. In most of these fanfictions, Edward might also be there, often having to deal with this new, dark and powerful side of Bella. Jacob could also be involved, sometimes as an ally and sometimes as an adversary to Bella's new self.
Definitely Naruto is a main character. There might be some antagonists that are drawn to the power of the Dark Phoenix within Naruto. These could be new villains created for the fanfiction. And Naruto's teacher Jiraiya might also be involved if the story delves into Naruto's search for understanding and controlling the Dark Phoenix power, as Jiraiya was always there to guide Naruto in the original series.
In 'King Dark Yaoi Fanfiction', the main characters usually revolve around King Dark, who is often portrayed as having a strong and imposing presence. The second main character is usually someone who is different from the king in some way. It could be a gentle healer who softens the king's hard exterior. Or it could be a rebellious thief who challenges the king's authority and in the process, a romantic relationship starts to develop between them.
It depends on the specific fanfiction. However, often there are characters that are associated with the themes of 'dark blue' and'moonlight'. Maybe a brooding hero type who is often seen in the dark blue of night or a mysterious figure that appears under the moonlight.