A believable character also needs to have a goal. In a Percy Jackson Hellsing fanfiction, a character could have the goal of finding a way to combine the powers or knowledge from both worlds. For instance, a demigod might want to learn the secrets of the Hellsing organization's vampire - slaying techniques to use against more powerful Greek monsters. Their actions and decisions throughout the fanfiction should be driven by this goal, which makes them more three - dimensional and believable.
First, start with Percy being in denial about his new identity as the son of Arachne. Then gradually have him accept it as he discovers the good aspects of this heritage. For example, the power and loyalty of the Arachne - related demigods.
Connect their existence to the existing lore of the Percy Jackson universe. Maybe their parent is a minor god or goddess that hasn't been explored much. Their powers and abilities should fit within the framework of what we know about demigods and the magical world in Percy Jackson. For example, if their parent is the god of dreams, they might have the ability to enter and manipulate people's dreams.
First, give your character a clear backstory. If they're an SI OC, think about how they ended up in the Percy Jackson world. Were they summoned? Did they fall through a portal? Their backstory should tie into their personality. For example, if they had a tough life before arriving, they might be more cautious or distrustful at first. Also, consider their powers. Make sure they're not overpowered right away. If they have a water - related power, start with basic skills like being able to sense water nearby and gradually develop more complex abilities as the story progresses.
First, give the OC a distinct backstory from their past life. Explain how they died and what their life was like before reincarnation. This will help in shaping their current personality. Second, tie their reincarnation to the Percy Jackson lore. Maybe they were reincarnated because of a curse or a blessing from a god. Third, give them a believable set of powers. Don't make them too overpowered. For example, if they have a power related to water, show how it's different from Percy's power.
To create a believable OC, consider the setting of Hellsing Abridged. The world is full of vampires, ghouls, and powerful organizations. Your OC should fit into this world. They could have a connection to an existing faction, like being a rogue agent of Hellsing or a new recruit for the bad guys. Their appearance should also match the tone of the series. Dark, edgy, and maybe a bit of a gothic look if they are on the side of the vampires or vampire hunters.
First, give your OC a solid backstory. For example, if they're a demigod, think about how they discovered their powers and how their mortal parent dealt with it. If they grew up in a difficult situation, like being bullied for being different, it can add depth. Second, give them a distinct personality. Are they brave like Percy, but also a bit of a loner? Or are they more of a comical character who uses humor to hide their insecurities? Finally, their abilities should fit within the Percy Jackson universe. Don't give them something too overpowered that would break the rules of the world.
Let the characters grow and change through their interactions. If Harry and Percy become friends, Harry could learn about Percy's world and become more open - minded. Percy could also be influenced by Harry's sense of justice. This development in their relationship would make their interactions more engaging and believable.
To create a believable relationship, start with a situation where they are forced to interact, like being thrown together in a dangerous situation. They can start by being cautious of each other, since they come from very different backgrounds. Percy might be wary of Jacob's werewolf nature as it's different from the Greek monsters he's used to, and Jacob might be unsure about Percy's demigod powers.
First, study the existing magic system in Percy Jackson. Make sure your OC's magic fits within the rules. For example, if most magic is related to the Greek gods, your OC's magic should have a connection too. Second, give your OC limitations. No one should be all - powerful. Maybe their magic drains them if they use it too much. Third, develop a backstory for how they got their magic. Were they born with it? Or did they get it through some sort of encounter?
You could create a character who is a descendant of an ancient being associated with the Styx. For example, a half - human, half - river spirit. This character could have unique abilities like being able to manipulate the waters of the Styx in ways that Percy can't. They could also have knowledge of the river's deepest secrets, which would be useful in the story.