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, giving you information on the owner of the chalice and how to steal it.
His name is a reference to Robin Hood, a thief who stole from the rich and gave to the poor in English folklore. He also states that he stole from the rich to give to the poor, namely himself.