He was voiced by Shane Richie. In the Japanese dub from the movie, he was voiced by Akihiko Ishizumi.
Sid is a sewer rat who arrives in Kensington out of the drain and he disappointed Roddy when he decides to stay.
Roddy attempted to get rid of Sid by flushing him down a toilet claiming it was a jacuzzi. However, Sid realized what was going on and retaliated by pushing Roddy into the toilet and flushing him into the sewers.
Sid is seen again when Roddy returns to his home with Rita Malone and he tried to pass Sid off as his brother Rupert before it is revealed that Sid and Rita know each other.
Later when Roddy realizes The Toad's plan is to drown the rats during halftime during the World Cup (as that would be the time the viewers and players would use their toilets), he has Sid flush him down the toilet again to get back into the sewers as everyone was in terrible danger, but not before letting Sid take his place at the house.
Sid is last seen at the house when Roddy's original owner returns home from her holidays with a cat causing Sid to be scared.
Sid was originally going to be voiced by Ricky Gervais but he left at some point during pre-production, giving the role to Shane Richie.
Its possible Sid was the rat Rita's mother mentioned earlier in the film. She says she once met one of Roddy's kind (an "up top" pet) who belonged to an old lady, implying the rat was flushed or ran away to the sewers once the old lady died (as she said "used to be some old lady's pet). Rita's mother mentions how lonely the rat was as he had nobody to talk to-Sid also interacts with the dolls much like Roddy did. He also seems to know a lot of surface things, such as their food, television, toilets, etc. When Roddy returns to the surface and introduces him as "Rupert", his brother, its shown that Sid and Rita know one another.
He also knows what a cat is as seen at the end of the film and has a fear of them. Its possible its all instinct.
Its possible all the things he seems to know about on the surface he's seen in the sewers, but he doesn't seem at all scared of the top world nor does he seem to want to go back.