I don't know who Toby is specifically in this story without reading it. He could be a child, a little animal, or even an adult with a child - like sense of wonder during Christmas.
Since the title only gives us the name Toby in relation to a Christmas story, he could be a variety of things. He could be a kind - hearted person who spreads Christmas cheer around his town. Maybe he's a lonely soul who finds warmth and friendship during Christmas. He could also be a naughty character who has a change of heart during the holidays.
I'm not entirely sure specifically what that fanfiction is about. It could potentially be about Bill Cipher achieving or obtaining Will Cipher in some way, perhaps in a strange or otherworldly scenario within the context of the Gravity Falls universe. It might involve some sort of power play or a very unique plot device that only fans who have read it would know for sure.
Toby could be anyone. Maybe it's a friend of the speaker, someone in a writing group, or just a fictional name used to make a general point about writing novels. Without more context, it's hard to say exactly who Toby is.
Well, in some fanfictions, Bill might kidnap Will to gain more power. Maybe Will has some special ability or knowledge that Bill wants to use for his own evil plans.
There could be a psychic character. This person might be able to sense things that Toby can't and help him understand the nature of the paranormal activities. Another character could be Toby's best friend who is skeptical at first but then gets drawn into the strange events.
Will might find a way to escape on his own. Since he is also a Cipher, he could have some hidden powers or tricks up his sleeve. Maybe he manages to outwit Bill and gets away, which then sets up a cat - and - mouse chase between them throughout the fanfiction world. The story could also take a darker turn. Bill might start to torture or manipulate Will, changing Will's personality or making him do things against his will. This could lead to a conflicted Will, and the other characters might have to deal with this new, changed Will when they finally find him.
One possible plot could be that Bill Cipher needs Will Cipher to complete a dark ritual. So, he devises a complex plan to capture Will. He might use his mind - bending powers to trick other characters into leading Will into a trap.