That shadow is most likely the one of 14th Noah - the Musician.Quote:
and remember the shadow that only Allen can see, I think he is the "heart" or I might be wrong.
The shadow is definitely related to the 14th; he appeared in the 14th's room, and has only spoken about Allen/Timcanpy in reference to them being the "musician" (or whatever).
Right now, there isn't enough evidence to pick one or the other; there are arguments for both sides, and even a third person. We'll just have to wait and see what happens... obviously, the Heart isn't going to be forgotten and become a plot hole.