It's possible that Jiraiya could be a main character. He's Naruto's mentor, and in the context of Naruto having a curse seal, Jiraiya would be involved in helping Naruto either control it or find a way to get rid of it. So he would play an important role in the fanfic.
The curse seal often gives characters increased power. For example, they might become stronger physically, able to take on tougher opponents. In some fanfiction, it can also change their appearance, like making their eyes glow or their skin take on a different hue.
One interesting storyline could be Naruto's struggle to control the power of the curse seal. Maybe he has to face inner demons and the temptation to use the dark power for quick solutions in battles.
One interesting plotline could be how Naruto learns to control the power of the curse seal. Maybe he initially struggles with it, but as the story progresses, he discovers unique ways to use it for good. For example, he might find that the curse seal gives him an edge in battles against powerful enemies, like Orochimaru's minions.
Well, the main character is likely to be the seal master. This person is probably highly skilled or on a journey to become highly skilled in the art of seals. There may also be other characters from the Naruto universe who interact with the seal master, like Naruto himself. Naruto could be a friend or a rival to the seal master, depending on how the story is written.
Naruto would likely be one of the main characters since it's in his name. He would be at the center of the story dealing with the shattered seal situation.
The curse seal can be a catalyst for Naruto's character development. It could force him to face his inner demons. Since Naruto is known for his positive attitude, the curse seal might introduce a darker side of him that he has to learn to control. This internal struggle can make his character more complex.
Anko is likely to be one of the main characters since she is in the title. There may also be other Naruto characters like Naruto, Sasuke, or Kakashi who could be involved depending on the direction of the fanfic.
There could be a new character introduced, perhaps an old hermit who is an expert in sealing. This character could have some hidden motives, like using Naruto's newfound skills for his own gain at first but then eventually becoming a true ally after seeing Naruto's pure intentions with the power of sealing.
Sasuke is often a central character. Given his own struggle with hatred in the original Naruto series, fanfics might further explore his journey in the context of the curse of hatred, perhaps showing how he tries to overcome it in different ways.
Since I haven't read this fanfic, I can't say for sure who the main characters are. They could be original characters created by the fanfic writer, or they could be characters from the source material that the fanfic is based on.