Argo "The Rat" serves as an information broker and courier in the Sword Art Online death game. Kirito and Argo suspect each-other without doubt to be a fellow beta testers but refuses to ever confirm the topic. As a broker she is known to sell any information for any price, including the story of her mysterious trademark whiskers. While she sells information to some at occasionally hefty prices, she is also known the be the author of free guide books situated in most town shops to help beginners.

Spoiler

Argo's whiskers were originally drawn on to learn the "Martial Arts" skill during the beta test. Unable to complete the quest before beta testing ended, they became her trademark and source of her nickname.