Beckett is obviously one of the main characters as the one with the mystery illness. There might also be a doctor character who is determined to find out the cause of the illness and treat it.
In some fanfics, there could be a mysterious stranger. This stranger might seem to know more about Beckett's illness than anyone else. Maybe they have a connection to the origin of the illness or they hold the key to the cure, but their motives are not clear at first.
Often, there will be a close friend or a romantic partner of Beckett. This person could be the one who stays by Beckett's side throughout the ordeal, providing emotional support and also helping in the search for answers. They might be the one who discovers some crucial clues about the illness.
Obviously, Beckett would be one of the main characters. There might also be her partner, if she has one in the fanfic's storyline. Her doctor or medical team could also be main characters as they play important roles in dealing with the complicated pregnancy.
Well, it could be that Beckett's mystery illness is actually a curse. In the fanfic, there might be a magical or supernatural element. For example, an old family feud with a witch or a wizard who put a curse on Beckett, and the story could revolve around breaking the curse to cure the illness.
A coworker who has a crush on Beckett might be a main character. They could be the one who stays by Beckett's side the most during his recovery, and this could lead to a romantic sub - plot.
Obviously, Beckett would be the main character. But there could also be a hotel concierge who plays an important role. The concierge could be someone who is either very helpful to Beckett, guiding him around the hotel and the local area, or someone who is a bit of a troublemaker, causing difficulties for Beckett during his stay.
The main characters are obviously Beckett and Josh. Beckett seems to be a character who is involved in some sort of relationship that they are about to betray through cheating with Josh.
Obviously, Beckett and his daughter are the main characters. Beckett is likely to be a protective figure, using his skills and knowledge to try and get them out of the trapped situation. His daughter, on the other hand, might be the source of hope or inspiration in the story, perhaps with her own unique ideas to contribute to their escape.
The parents of the Beckett baby would surely be main characters. They would be in a state of panic and desperation, driving the story forward with their search for their child.
The main characters would be Beckett and Castle, as the title implies. Beckett is likely to be the one taking some sort of action in relation to Castle.
His doctor could be a main character. The doctor could be portrayed as a very dedicated and empathetic person, constantly providing Beckett with hope and medical advice. For instance, the doctor might have to break some tough news to Beckett about the risks of the transplant, but also encourage him to stay positive.
There's not enough information to say exactly who Beckett is. It could be a random person with that name who has had an impact on fanfic. Maybe it's a pen name or an alias of someone within the fanfic community or someone who has some sort of authority over the works that fanfic is based on.