Who is Mittenwool?

Silas calls Mittenwool a ghost, although he is not sure that is the right word for him. Silas's father thinks that Mittenwool is an imaginary friend. However, Silas says that Mittenwool is as real as the chair he sits on, the house they live in, and the dog. Only Silas can see or hear him, but that does not mean he is not real.