Assistant Serf is a NPC discovered after completing Enlightened Journey. Players can talk to him to use the hot air balloon system. He was a serf who was freed by Auguste and is now employed by him as the balloon wrangler near the Varrock Lumber Yard.
During, and after, the Middle Ages, it was customary for tradesmen to assume their professions' as their surnames (e.g., "Miller" -one who grinds meal; or "Sawyer" - one who chops wood). While "serf" is not a trade, per se, some other reference to prior employment, or famous activity, was also very common.