A Challenge, Naruto in One Punch-Man.
Saitama and Naruto have been childhood friends and eventually roommates. They've been around one another pretty much at all times. It only seemed logical that it would continue when they stepped into the life of superheroes and monster slayers. While Naruto's power comes from a mystical reincarnation of his former self...Saitama's...well Naruto doesn't really know about that.
Naruto has been living in this odd world for as long as he could remember. He thought it was a peaceful place, but monster's appearing out of nowhere and random people getting involved with him is annoying. Whether it'd be a beautiful woman trying to recruit him, or a scar-faced man wanting his protection.