The Bartender at the Blue Moon Inn in Varrock is a non-player character who serves the purpose of selling beer to players.
The bartender also seems to be one of the few NPCs who is aware that RuneScape is a "computer game". This is an example of a character "breaking the fourth wall".
When players enquire what the bartender means by this, he makes a brief attempt at explaining what a computer is, calling it a "magic box". However, considering that RuneScape is set in another world in a Middle Ages or Renaissance-like era, the player is baffled, and calls the bartender crazy (although players seems to acknowledge that RuneScape is a computer game while talking to Father Aereck, the priest in the Lumbridge Church).
The bartender also serves a purpose in level 1 Treasure Trails. In some clue scrolls, he may be the person to give you your next clue, or he may give you your reward.
During the Alfred Grimhand's Barcrawl miniquest, he is one of the Bartenders that you must get a drink from. His drink is the "Uncle Humphry's Gutrot". It costs 50 coins (you can only purchase if you are doing the Barcrawl) and lowers the player's Attack, Defence, Strength, Hitpoints and Smithing. It also makes you throw up.