There could also be Beckett's fiancé in the story. He is an important character as Castle is stopping Beckett's wedding to him. His presence might be the very reason Castle feels the need to act.
The main characters are Castle and Beckett, obviously. Castle is likely to be the one who intervenes in Beckett's wedding. Beckett is the one whose wedding is being stopped.
Maybe other characters from the original series, like Beckett's friends or Castle's family, could be involved too. They might play roles in either supporting or opposing Castle's attempt to stop the wedding. For instance, Beckett's friends could initially be against Castle but later realize he's right.