Comic Master J

Legend says if you write a message in the Bulletin Board of the Shibuya train station, a misterious man may help you finish your manga. Comic Master J is the legendary assistant that can finish a draft in no time regardless of the drawing style in exchange of 50.000 dollars. However, he will not work in your manga if it doesnt have a soul.
But… what is J's true identity? why does he wear an overcoat filled with drawing utensils? and more important… why is it that a man with such talent decided to move in the shadows of this industry instead of working on his own manga?