Well, it could be that in the context of the fanfiction, Clarise has a deep understanding of the threats Percy faces. She may have witnessed some near - misses or had premonitions about his safety. So, she becomes overprotective to prevent any harm from coming to him. Also, it could be related to some unspoken bond or connection they share within the story's unique plot.
Maybe Clarisse has her own reasons. She might see Percy as someone who needs protection in the complex world of the fanfiction setting. It could be that she has a soft spot for him deep down, or she feels a sense of responsibility towards him for some unspoken events in the story.
Clarise's overprotectiveness can affect the relationships in the story. It could strain her relationships with other characters who don't approve of her actions. At the same time, it might strengthen her bond with Percy, either in a positive way if he appreciates it, or in a negative way if he feels smothered. This complex web of relationships influenced by her overprotectiveness can drive the story forward in many different and unexpected ways.
One common theme is friendship. Percy's overprotectiveness often stems from his deep love for his friends. For example, he doesn't want to see them get hurt during battles or dangerous quests.
One common theme is friendship. Percy's overprotectiveness can be seen as an extreme form of his friendship with Nico. He wants to keep Nico safe from the various threats in their world, which shows how much he values their bond.
It could be that Percy has a special connection to one of the major gods. For example, if he's related in some way to Zeus or Poseidon, the other gods would be extra careful with him. Also, Percy might have done something in the past that made the gods feel indebted to him, and this overprotectiveness is their way of repaying him.
Well, 'Percy's Overzealous Protection' is another great one. It explores Percy's overprotective tendencies not just towards his close friends but also towards new demigods at Camp Half - Blood. He tries to shield them from all the perils of the demigod world, and the story unfolds with his efforts causing both admiration and some exasperation among the other characters.
One place to start looking could be Archive of Our Own (AO3). Just search for 'percy is overprotective of nico' in their search bar, and you'll likely find a good number of relevant fanfictions.
In the fanfiction, the overprotectiveness might show up as the gods restricting Percy's actions. They may not let him go on certain quests alone, insisting on sending a whole entourage of helpers. This could be because they see Percy as a very important asset, and they're afraid of losing him. So they overcompensate by being overly protective.