Overview
Ser Rodrik is the master-at-arms for Eddard Stark in Winterfell.
Background
Rodrik Cassel is a knight in the employ of House Stark. He is Winterfell's master-at-arms, and is thus responsible for the combat training of many of the Stark soldiers as well as the Stark boys and general safety/battle advisor. As mentioned, he is a knight, which is quite uncommon among the people of the North.
Major Plots
Rodrik is first seen relaying the news that a deserter from the Night's Watch has been captured to Eddard Stark, and silently watches as the deserter is sentenced and beheaded. After the failed assassination atempt on Bran, it is Rodrik who first identifies the assassin's blade as being made of valyrian steel, and offers to be Catelyn's guard when she journeys to King's Landing to warn Ned. The pair travel to the capital, and on their return to Winterfell they come upon Tyrion Lannister at an inn. There, Catelyn persuades the soldiers at the inn to capture the Imp, and Rodrik has his sword pointed at Tyrion's throat. After the Imp's capture, the group turns its heads to the Vale of Arryn, and the Eyrie. On their way there, they are attacked by savage mountain men, who they manage to fight off. Rodrik recieves a wound during the fight, but manages to carry on. While at the Eyrie he vitnesses Tyrion's Trial, and further accompanies Catelyn when she leaves and meets up with Robb.