Legend has it he's the guy who started ninjitsu and all the ninja crap. He was the first. I forget his name exactly, be he had the rinnegan. Some say he and the rinnegan are just a myth, but Pain proves otherwise. He apprently defeated the 10 tailed beast by sealing it inside himself and became the savior of the world or something.
Oh wait....here:
The Sage of the Six Paths (六道仙人, Rikudō Sennin) was said to be the legendary figure who founded the ninja world and was the creator and first user of modern ninjutsu, essentially making him the first shinobi in the modern context of the term, as well as the first jinchūriki.
http://naruto.wikia.com/wiki/Sage_of_the_Six_Paths