Catspaw assassin

The catspaw assassin is a minor character in the first season. He is played by guest star Lalor Roddy and only appears in "The Kingsroad." He is never named, but is referred to as such in casting announcements and the script, a reference to a catspaw being someone unwittingly used to further another person's agenda.

Season 1
The catspaw assassin was paid to kill Bran Stark whilst he is in a coma in Winterfell, following his fall. The assassin arranges for a fire to start in the castle, reasoning this will draw people away from Bran's room. This diversion succeeds in persuading Robb Stark to leave the room, but Catelyn remains behind. When the assassin goes into Bran's room to kill him, he finds Catelyn there. They struggle and he injures Catelyn's hand, but is then attacked by Bran's direwolf, Summer, who tears his throat out.

The assassin carried an elaborate dagger with a dragonbone hilt and a Valyrian steel blade. Catelyn resolves to carry the blade to her husband in King's Landing to investigate further. Petyr "Littlefinger" Baelish falsely claims that the dagger belongs to Tyrion Lannister. This leads Catelyn to taking Tyrion prisoner when she chances across him at the Inn on the Crossroads. Her actions spark a conflict between House Stark and House Lannister. Tyrion proves his innocence of the crime via a trial by combat at the Eyrie.

In the books
In the books the assassin is never named.