Argus Filch, the caretacker of Hogwarts School of Witchcraft and Wizardy. She follows her boss everywhere. If you are Filch, you can use Mrs. Norris as a special abillity. She has the abillity to dig.
Normaly, you can see Mrs. Norris everywhere in Hogwarts where Filch goes, but sometimes you can see her in levels. She don't appear in the DS version.