Kojiro's a very playful, laid back anomaly of a samurai.  He's seemingly determined never to spend a day in the dojo, instead choosing to learn from nature. He actually died as a young man but, in death, trained constantly to overcome the man who killed him Musashi Miyamoto, arguably history's greatest warrior. Because we see and hear his younger self quite frequently in the manga, we can't have his voice sounding too young, so do you best laid back, wise old man impression. Medium.