HOLY RAMEN NOODLES EVERYBODY! i just thought of a great theory. (at least i think it's great)
okay so i noticed that in the captivate demo part 1 in the very beginning when dante is walking the street with kat dante does not have rebellion. then when dante is pulled into limbo rebellion is magically on dante's back! this holds true again when dante is in limbo then he jumps out of the church window returning to the human world and rebellion vanishes, which got me thinking alot about rebellion.
so rebellion is a devil arm correct? and how are devil arms collected? by killing demons and collecting their reincarnated souls. it has been confirmed that sparda gave rebellion to dante, so what i'm thinking is that sparda killed a strong demon or perhaps a demon/angel hybrid and it's soul was reincarnated into the sword rebellion. now since rebellion is the soul of a demon or demon/angel hybrid it can transform into different forms such as the angel and demon versions of the weapon. i also think that we can only see rebellion when dante summons it, which he only does in limbo.
that is my theory on rebellion, now to further my theory i will talk about how i think yamato came into existence. this part of the theory also explains what i think most of the plot is.
okay so i am guessing that the voice we heard in the captivate demo footage is vergil's. my guess is that in this game vergil is a good guy along with dante, and kat is actually a demon in disguise. this would reference to dmc1's plot where trish appeared human but was actually a demon trying to trick dante. the same goes for kat, she is trying to lure dante deeper and deeper into limbo so he can be trapped or killed. eventually vergil catches onto kat's plan and vergil kills kat thus gaining her demonic soul, thus gaining a devil arm. and that devil arm happens to be yamato! that's my theory anyway.