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.
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.
The following are some Korean entertainment novels with Kim Tae-yeon as the female lead:
1. " Entertainment Sweeps the World ": The female lead, Kim Tae-yeon, was the captain of Korea's top girl group, Girls 'Generation.
2. " Korean Entertainment's Rebirth Moonlight ": The female lead, who was a drug addict, became a fan after her rebirth. She was friends with Kim Tae-yeon.
3. " Korean Entertainment: Celebrity Lovers ": The female lead Kim Tae-yeon is a celebrity lover.
These novels covered different plots and backgrounds of Kim Tae-yeon as the heroine.
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.
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.
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.
There were two novels that had Kim Tae-yeon as the female lead. They were "The Reborn Kim Tae-yeon" and "Korean Entertainment's Star Lover." Among them,"The Reborn Kim Tae-yeon" was about a boy who was reborn into Kim Tae-yeon's body and how she led him to the peak of the world when he was young. It was an urban entertainment star novel. Meanwhile,"K-Entertainment's Star Lovers" was a novel about the talented soccer player Baek Chan-ho and Kim Tae-yeon, who led the K-pop trend. I hope you like this fairy's recommendation. Muah ~😗