He came upon Vex being attacked by a pack of wolves and assisted her in killing them. Immediately afterward, Vex attempted to rob him. They were locked in combat, but neither could best the other. Impressed, Vex invited him to join the Guild.
It's also hinted that Garthar has taken a small liking to Vex when he says, "I wouldn't want to cross blades with Vex again, but I certainly wouldn't mind a bit of a scuffle."
He will join the Thieves Guild after the Dragonborn completes three special jobs for the guild. Garthar appears outside of the Ragged Flagon - Cistern when bringing Karliah to the Guild. Despite being a Nord, he is taller than any character in the game, including High Elves.
- The Elder Scrolls V: Skyrim (First appearance)