All characters other than a main character or characters are side characters from Literary perspective. So, given Naruto's context, only Sasuke and Naruto are main characters, no one else. All characters' have importance and plot value secondary to them. In fact, if one considers the Core development, Sasuke is the main character in that regard. Had the manga not delved into Naruto's past, the manga - from literary perspective - despite being called Naruto, would have pushed him to the secondary status itself.
I still consider Sasuke to be more relevant, given that even Naruto's development has happened because he was always the key instrument. It's only with this destiny logic that Kishimoto has tried hard to squeeze him in.