As both the narrator and protagonist, Axel Lidenbrock is our passageway into this adventure story.  Verne uses Axel to shift the tale into a bildungsroman, a coming of age story, and not just a quest piece.  Axel is young and romantic, but begins the story with a cynicism that is the foil of his larger-than-life uncle.  He shows this this cynicism through his reluctance to even admit what he has discovered to his uncle, terrified that once Otto knows, there will be no holding him back:

“Ah!" I cried, springing up. "But no! no! My uncle shall never know it. He would insist upon doing it too." (Chapter 4)

In contrast to this fearful resistance of adventure is Axel's very romantic sensibilities.  We learn early on about his secret engagement to his uncle's ward:

"The pretty Virlandaise and the professor's nephew loved each other with a patience and a calmness entirely German." (Chapter 3)

This passion, and his decision to secretly engage, shows he is more adventurous than he admits.  Throughout the story, and the quest, he will be influenced by his uncle's zest for life and discovery.  While his uncle was once a burden, seen as "irascible," Axel's experience causes his to mature to honor his uncle:

"Then my uncle became a great man, and I was now the nephew of a great man -which is not a privilege to be despised." (Chapter 45

Axel embraces his adventure, writing the story for world to read, and marries his love.  He has, in fact, fully matured as a result of his journey.