A lawyer could be an important character. This lawyer takes on Alexis's case, believing in their innocence. The lawyer has to battle a corrupt legal system or powerful opponents who want to see Alexis locked up for good. The fanfic could show the lawyer's strategies and how they build a case in Alexis's favor.
Well, it could be that Alexis was wrongly accused of a crime. Maybe a jealous rival framed Alexis for theft or something. Then the story could follow Alexis's experience in jail, how they try to prove their innocence and find the real culprit.
Well, clearly Jaden and Alexis are the main characters. They are the focus of the fanfic.
Obviously, Alexis and Castle would be the main characters. Their relationship as father and daughter is central to the 'Castle' series, so in this fanfic, their dynamic would be crucial. Castle would be the one trying to find Alexis, and Alexis would be the one in the middle of whatever situation led to her disappearance.
Perhaps there have been some rumors or whispers in their fictional world that have reached Alexis' ears. Even if they are baseless, they can plant the seed of doubt in Alexis' mind, making her think that Kate is cheating.
Alexis might confront Beckett directly. She could say something like 'I think you're cheating on me. What's going on?' This is a very straightforward approach.
It could be that Alexis is being influenced by someone else. There might be a third character who has their own agenda and convinces Alexis to go after Beckett. Also, Alexis might have some internal issues, like self - esteem problems. If Beckett is constantly achieving things that Alexis feels she can't, it could lead to feelings of inadequacy and then the urge to bring Beckett down.
The two main characters are Jaden and Alexis. They are at the heart of the fanfiction. Maybe Jaden is a strong, protective type, while Alexis is more gentle and nurturing. Their personalities will likely play a big role in how they handle the pregnancy and the events that unfold in the story.
I don't know who Alexis is in this story without reading it. But it's likely that Alexis is either the main character or an important figure in the events that unfold in the 'creepy subway' setting.
I have no idea who created the 'alexis sartorius moan fanfic'. It could be an individual fan with a lot of creativity and a specific interest in the character Alexis Sartorius.
She could be defensive of Beckett. Since Alexis probably has a good relationship with Beckett, she may try to convince Castle that he's wrong and that Beckett didn't mean any harm. For example, she might tell Castle about all the positive things Beckett has done for her.