It's important to keep in mind that in C, the three types "char", "unsigned char", and "signed char" are all distinct types, even though "char" has the same format as one of "unsigned char" or "signed char." You can freely assign between these types, because the language allows implicit conversion between arithmetic types. However, the language does not allow implicit conversion between pointers of different types.
I assume you intend to use the typedef BYTE to do byte-wise access of larger objects. In that case, you should define it as "unsigned char," because the standard guarantees you can do this with "unsigned char", but does not necessarily guarantee you can do it with "signed char" or "char."