It was 1953 when a man named Argus Filch applied for a job at Hogwarts school of Witchcraft and Wizardry. The students from that day onwards found him to be rather an odd man. There was a cat which followed him everywhere which nobody had ever been able to understand. Nobody except the select few including Minerva McGonagall, who found out through rather strange means, and a boy called Peter Pettigrew who attended Hogwarts for a few years during which time he made became good friends with this cat, not that anyone else knew that of course.
On the application form, Filch had asked if there would be any chance that his cat could accompany him to the school and, of course with Hogwarts being Hogwarts, they had agreed to allow him to take his cat. They didn't expect his cat to live for quite as long as it had done though. 50 years later and the poor old thing was still following him around. Of course, by now though she had aged considerably.
Minerva McGonagall had started working at Hogwarts about 5 years after Filch and his cat first arrived there. Of course, with her being an animagus, she was able to transform into a cat and have long conversations with her. And about 10 years later, Peter Pettigrew joined Hogwarts and he also was able to have conversations with her once he had learnt to transform into a rat. Cats and rats were hardly ideal friends but hey, neither of them particularly cared.
Mrs Norris had to be one of the friendliest cats Peter Pettigrew ever encountered and Minerva McGonagall got a friend out of it as well which meant that Mrs Norris was rarely lonely. There were things which nobody knew about Mrs Norris and things that only the two of them knew about her as well. Yet the tales they could all tell about one another were something of a mystery to anyone else unless they understood, as was the case with Sirius and James, two of Peter's friends. Although he never spoke about it to anyone.