Who is Grauben?

As Otto Lidenbrock, Axel, and Hans descend further into the earth, they encounter a truly awesome spectacle—a gigantic crater. As far as the eye can see, the men are surrounded by features of the natural world that strongly resemble those on the earth's surface, except that they are much, much bigger. Axel is astonished by a large forest full of giant mushrooms thirty to forty feet high. The party soon set about building a raft out of wood hewn from the massive petrified forest. They then set sail on the vast subterranean ocean. Otto modestly christens the ocean the "Lidenbrock Sea" and names their point of embarkation "Port Gräuben" after his goddaughter. Gräuben is also Axel's sweetheart, and he will marry her once he returns to normality from his epic adventure.