Maybe Percy has been through a great betrayal, like his closest friend turned on him for power. So he starts to close off his emotions to avoid being hurt again.
Perhaps he witnesses a great tragedy that he couldn't prevent, such as the destruction of his home. This makes him cold - hearted as he feels that being emotional won't help and he needs to be tough. He might start to push people away and only focus on his own goals, regardless of how it affects others.
A love interest who breaks his heart multiple times. For example, if this person plays with his feelings, lies to him, or cheats on him, Percy could become cold - hearted as a form of self - protection. He would no longer trust easily and would close his heart to new relationships.
One possible plot could be that after losing many friends in a major battle, Naruto becomes disillusioned. He might start to see the world as a cruel place where strength is the only thing that matters. So he shuts off his emotions to avoid further pain, and becomes cold - hearted towards his former allies and enemies alike.
Another idea is that Percy discovers he has a long - lost vampire heritage. As he becomes a vampire, he starts to unlock memories and powers related to his vampire lineage. He could go on a journey to find out more about his vampire ancestors and the secrets they held, all while adjusting to his new vampiric existence which is full of both challenges and newfound abilities.
In this fanfic, Percy might be seen as an outcast at first among the Avengers because he's so different. But as time goes on, they come to respect his abilities. The story could also explore how Percy's presence affects the dynamics within the Avengers. Does he create new rivalries or alliances? And how does he adapt to their high - tech world when he's used to the more ancient and magical world of the Greek gods? Maybe he even tries to introduce some of his own magic into their technology, leading to some interesting and unexpected results.
Perhaps Percy is pushed to his limits by some great injustice in the story world. Maybe he witnesses the death of someone he loves at the hands of an evil force. This trauma could transform his personality, leading him to become a killer with a vendetta. He might start by targeting the ones directly responsible and then expand his scope as his thirst for vengeance grows.
Well, perhaps Percy discovers some ancient power within himself that gradually transforms him into a god. He might have to face a series of trials to fully ascend, like battling powerful monsters or making difficult moral choices.
Maybe Percy discovers a hidden garden and becomes obsessed with it. He starts to learn about different plants, takes care of them, and the garden becomes his special place.
One possible plot could be that Naruto has faced so many betrayals. Maybe his friends turn on him due to some false accusations. This shatters his warm - hearted nature, and he becomes cold as a defense mechanism. He starts to isolate himself and no longer trusts easily.
In fanfic, a tragic backstory that was different from his original one could lead to Naruto being cold - hearted. Maybe if he had been abandoned by the whole village instead of just being shunned by some, he could grow up bitter and cold. Also, if he had to make really tough decisions that cost him a lot, like sacrificing friends for the greater good in the fanfic world, it might harden his heart.
A possible plot could be that Percy has a personal experience that makes him want to become a police officer. Maybe he witnesses a crime and feels helpless. So, he decides to take matters into his own hands. During his training, he befriends some fellow trainees, and together they form a tight - knit group. Once on the job, they are given a cold case to solve. Percy uses his unique perspective and determination to crack the case, which had been baffling the department for years. This earns him the respect of his colleagues and the community.