Do filler episode characters ever appear in the main storyline?
Utakata was introduced during the so-named Tsuchigumo Kinjutsu arc, but he eventually makes his way in the main storyline as a reanimated ninja fighting in the Great Shinobi War.

It could be conjectured that he wasn't a filler character, but until Naruto chapter 515 at the earliest, the Six-Tails host was not identified by name.



In this picture, Utakata is second from the left.