Ok if you are a real Naruto fan you would not care if there are fillers as long as you are able to watch Naruto. Personaly i dont care if there are fillers because Naruto is my favorite show and i would watch 1000 fillers.
Please, for the love of all that is holy, stop with the fillers! I understand about character development but d*mn, it feels like we've been watching fillers for about a half year!!! WTF Kishi??? Please get the story rolling again, I beg of you!
The Eternal Genin, Kosuke, first appeared in Naruto Shippuden episode #190, as a flashback episode at the end of the Pain Saga. At this point, he's an Anime-Only character. Whether he makes it into the manga or not is pretty unlikely.
He decided to remain a Genin for life, because some of his friends died because of a battlefield decision he made. It's never shown what decision, just that people died. It probably took place during or before the First Ninja World War, since the Second Hokage was the ruler of the village at the time.