Who or what is the wolf in ff7 advent children?

If you are talking about the grey wolf following Cloud then he is there to represent the guilt Cloud feels. There is also the theory that it is the spirit of Zack, but either way, he's there because Zack is not. If you are talking about the red wolf with one line of speech at the end ('There are still children with the stigma') then that is Red XIII, a main character you find in shinRa's lab. You should know that my friend =p ^_^ xx