You can develop Carrie's character by showing how she reacts to different situations with her powers. For example, when she's angry, does she lose control easily? Harry can be developed as a bit of a rebel, but with a good heart. He might encourage Carrie to be more assertive and not let others push her around, which in turn helps Carrie grow as a character.
To develop Carrie, describe her relationship with her mother and how that affects her. Since she has powers, she might be constantly worried about her mother finding out. Harry can be developed as someone who is popular at school but still values his family. He could introduce Carrie to new friends in a gentle way, helping her come out of her shell.
For Carrie, you can focus on her internal struggle with her powers. Show her moments of fear, confusion, and also the times when she starts to feel a sense of control. For Harry, you could make him very curious and protective. He might be the one who is always asking questions and trying to keep Carrie safe.
Another plot could involve a mystery in their family's past that Harry and Carrie uncover. They might find out that there are other relatives with powers too. As they dig deeper, they face some sort of danger from an unknown source that is related to their family secrets.
They often give the characters distinct personalities. One cousin might be more rebellious, while the other more reserved. This contrast can create interesting dynamics in their relationship.
Obviously Carrie White herself. And perhaps some of the characters from Derry's own stories, like the Losers' Club members could make an appearance. They might interact with Carrie in different ways, either as allies or as those who are initially suspicious of her.
One common theme is redemption. Since Carrie's story in the original is so tragic, many fanfics try to give her a chance at redemption, like making amends for the destruction she caused.
Their relationship could start off as a bit of a cautious friendship. Gumball might be a bit wary of Carrie's spooky side at first, but as they spend more time together, he realizes she's really kind. And Carrie could see Gumball's fun - loving and adventurous spirit and be drawn to it. Eventually, they become really close friends who support each other through all kinds of crazy situations in Elmore.
In certain 'Harry Ginny after the war fanfic', Harry starts to see the wizarding world in a new light. He's no longer just focused on fighting dark forces. Ginny encourages him to explore the positive aspects of the wizarding world, like the different cultures of magical creatures. Ginny also matures in how she handles relationships. She becomes more understanding of Harry's responsibilities, and they work together as a team to face any new challenges that come their way, whether it's dealing with a new threat to the wizarding world or just the normal challenges of married life.
It could start with animosity. Caius might see Bella's cousin as a potential threat or just an annoyance at first. But then, as he gets to know her better, he realizes she has a kind heart. He starts to protect her from other vampires or dangers in Forks, and slowly, feelings of love or at least deep affection develop.
In these fanfics, Harry often develops from a somewhat naive boy into a more confident and powerful wizard. His relationships with the harem members force him to grow up faster. For example, if one of the harem girls is from a different magical culture, he learns about new spells and traditions from her.
In a little Harry Potter fanfic, developing characters can be fun. Look at the existing characters in the series for inspiration. For example, if your character is a Gryffindor, they might have some of the same traits as Harry, like bravery, but also have their own unique quirks. You can show character growth through the challenges they face. If they're afraid of heights but have to face a flying lesson, how will they overcome this fear? This kind of internal struggle can make the character more interesting.
One way is to give them distinct personalities. For example, one could be brave and impulsive like Harry, while the other is more reserved and intellectual like Hermione. Consider their magical abilities too. Maybe one is a natural at Transfiguration and the other at Potions. Also, think about their relationships within the fanfic world. Do they have different groups of friends? Are they on good terms with the Weasley twins? Their backstories can also play a huge role in shaping who they are. Were they raised in a wizarding family or were they Muggle - born and just discovered their magic?