Chief Thief Robin

Chief Thief Robin is a member of Lumbridge's Thieves' Guild and second-in-command there. When met, he'll tell you to speak to his boss Darren Lightfinger. In his life prior to the Thieves' Guild, he was a poor man living in Lumbridge. After he joined the Thieves' Guild he started stealing from the rich.

Buyers and Cellars
During the Buyers and Cellars quest, Robin is your main contact person, telling you the owner of the chalice.

Trivia
When asked about how long he has known the guild master, he references the Guildmaster previously being a con artist, stealing one's valueable, and vaguely promising rewards.