Hagoromo Ōtsutsuki

The was said to be the legendary figure who founded the ninja world and was the first to invent Ninjutsu. He was said to be the only shinobi to possess the Rinnegan, which presumably laid the groundwork for the other two great Eye Techniques, the Sharingan and the Byakugan. He is revered as one of the most legendary people in the Naruto Universe, and is likely one of the strongest ninja that ever existed.

As of now, the only other person to be known to possess the Rinnegan is Nagato (Pain), an orphan from the Rain country. Whether this means that the Rinnegan is a unique Kekkei Genkai, that isn't passed down by blood, or if Nagato (Pain) is directly related to the Sage of the Six Paths is currently a mystery.