I believe a strict definition of “filler” should describe an episode that doesn’t make any significant development in character, plot, setting, or theme. However, common fandom parlance throws around the label for episodes that are unrelated to the main plot and are of perceived lesser quality. But in either definition, “Sympathy for the Devil” doesn’t qualify because of its strong emphasis on character, setting, and theme, and the masterful workmanship of Bebop that’s evident in all of its episodes. There is no valid justification of calling Bebop filler; it’s either a thoughtless attempt to detract the work as a whole, or someone genuinely believes Bebop would be better off as a six episode, plot-centric series. Those people can have that show if they want it, but I’ll take my sweeeet harmonica riff and enjoy my show.