He really was until a few chapters ago... Sure, during the flashback arc he definitely wasn't a minor character but that was a looooooong time ago, things change. For a large part of the manga Bunshichi wassn't doing anything to effect the plot.
Tjtg in general is filled with lots of badass side characters. Bob began with a chance to be a prominent character and slid away into the darkness(pun not intended). The real main characters of Tjtg are Aya, Maya, Nagi and Mitsuomi.