User blog comment:TonyMutu/Thoughts on 2003 anime/@comment-23863-20130108172506/@comment-2124439-20130110230443

By the way, "filler" are episodes that are entirely skippable. Isaac McDougal is referenced in many future episodes, so, the first episode is definitely NOT filler. It's an anime exclusive, yes, but it still fits into the canon of the rest of the story.