Just a regular person here.
Edited to match Viz's publication.
There are limitations on how I could write the character because he was a "symbol of justice," so he may be a little on the boring side. I solidified his character as I went. Jonathan is passive, reacting to Dio's various attacks, and this leads to him discovering his way of life. Perhaps this is linked to me as an author, growing along with my character as I drew him. Just as Jonathan was unsure as to how to live his life, I was unsure as to where to take the character. Maybe I grew as an author a little with Jonathan as he trudged on through his hardships.
In Part 1, during the seven years after Danny's death, Jonathan gets very muscular. This change was made with the upcoming battle between him and Dio in mind. I thought his physique needed to be able to withstand the constant onslaught from this point on. In addition, when this part was originally being serialized, it was the era of "muscles" on the silver screen—guys like Schwarzenegger and Stallone. Schwarzenegger, for example, could never be stopped by any amount of gunfire, right? I wanted Jonathan to have a similar look to him—a look of unstoppableness. Weekly Shonen Jump also has a history and tradition of main characters evolving and growing.
I also wanted Jonathan to exude an aura of strength like in Karate Baka Ichidai after doing martial arts training alone in the mountains. As a child, I loved karate manga—you would see all kinds of supernatural strength, like punching electrical poles and having the birds on the wires drop down or someone trying to stab another person with a knife, only to have the knife bend. In actuality, martial arts does have a certain mystique to it, such as strengthening punches with breathing techniques. I wanted to include supernatural things like that in JoJo, and I believe that the Hamon is inspired by my love of karate manga.
Did Dio and Danny make up?