Even the antagonists in the story, if there are any, can be interesting to write about. Maybe they try to break up the main couple for their own reasons. You can explore their motives and how the couple overcomes their attempts to split them up.
The friends of the main couple can also be great to focus on. They can add humor, support, or even create some conflict. For example, a friend who is always teasing the couple about their relationship can be a fun addition to the fanfiction.
Most likely the main characters would be the ones from the original 'it started with a kiss' series. They would be the ones whose relationship started with that kiss and are now facing 'the last kiss' situation.
In some fanfictions, there might be additional side characters that play important roles. These could be friends or family members of the main couple. They might influence the relationship in different ways, like giving advice or causing misunderstandings.
The main family members can be great to focus on. Their relationships, power struggles, and dreams offer a lot of material. For example, the patriarch of the family who has to make tough decisions to keep the ranch going. Another good character could be the strong - willed matriarch who holds the family together. Also, the young and rebellious heir who wants to do things his own way.
Kaden is another character that many like to write about. His loyalty and his skills as a guard give a lot of scope for creating stories. You could explore his relationship with Lia in different ways, or even create scenarios where he has to make difficult decisions that test his loyalty.
Sersi is a great character to focus on. She has a strong connection to humanity and her powers are quite interesting. You could explore her relationships with humans in more detail. Another good one is Ikaris. His loyalty and the internal conflict he might face are good material for a fanfiction. Then there's Kingo, with his love for movies and his unique charm among the Eternals.
All of the members of 4 town are great to focus on in different ways. Tae Young, for example, might be the subject of a fanfiction where his amazing dancing skills are the center of the story. Maybe he gets an offer to be in a big dance competition outside of the band, and he has to decide whether to take it or stay loyal to 4 town. Each member brings their own unique traits that can be explored in fanfiction.
Since I haven't read the 'it started with a kiss novel', it's hard to say for sure. However, in such a novel, there are likely to be two main characters, perhaps a young man and a young woman. The male character might be charming and a bit of a risk - taker, while the female character could be more reserved but has a strong will. Their personalities would play a big role in how their relationship, which started with a kiss, develops throughout the story.
Sebastian is a fun character to focus on. You can write about his loyalty to Ariel and how he tries to keep her out of trouble. He could have his own mini - adventures in a fanfiction, like getting lost in the human world while trying to find Ariel. Flounder is another good one. You could show his growth from a timid fish to a more brave friend of Ariel's. Also, Triton, Ariel's father, can be a central character. You can explore his relationship with Ariel in more detail, like his struggle to let her go.
In my opinion, Magnus Bane could be a great character to focus on in Clockwork Angel fanfiction. He has such a flamboyant personality and his magic abilities are quite interesting. He could interact with the main characters in ways that haven't been explored in the original story. Another good character would be Charlotte. As the head of the Institute, she has a lot of responsibilities and secrets that could be delved into. And of course, Tessa. She is the center of the story in many ways, and her character development is something that can be further explored in fanfiction.
Obviously, Bella herself is a great character to focus on. She's the main character and has so much potential for different storylines. You could explore her growth as a football player and as a person.