Who is Ser Dontos from A Clash of Kings and what is their importance?

Ser Dontos from A Clash of Kings is a very old very drunken knight that falls off of his horse in the middle of a tournament being held for King Joffrey's birthday. This insults Joffrey who orders him to be drown in a cask of wine for his disrepsect. Sansa talks him out of it by saying that it is bad luck to kill someone on your birthday.

He is so grateful to Sansa that he conspires with her to sneak her away from the castle. They arrange to get away during the distractions of Joffrey's wedding celebration.