Gustav Kube is a Magician who explains the city of Geffen to the the protagonists. His character model would later be used as the basis for the Magician in Ragnarok Online. It's also possible that his facial expressions were used for the game's various emoticons.
While wandering around Geffen with Iris and Loki, Lidia comes across Gustav who enthusiastically informs them about the city and the Magic Festival. He leaves the group shortly after and was seen winning a round in the Tournament of Magic.
Gustav and his staff.
Gustav says goodbye to the group.
Gustav shows off his Magician skills at the Tournament of Magic.