Wow, that's very good, one of your best pieces ever, I would say. It's obvious that you like the character with the attention you paid to detail, especially when matching the colors. I particularily like how you did the highlighting on his bangs. The way you do eyes makes him look younger than GB cannon, but all that makes me think of is the flashbacks to chibi Kazuki and Jubei, and all of the wonderful squishy goodness that was.

The shading on his shirt seems to be a light grey color, but when shading white like that, you can sometimes use a very light, almost white blue color on the folds. That should be helpful should you choose to draw a full body picture someday.

Now that you've drawn Kazuki, Ban and Ginji, do you have any plans to draw more GB characters?