Active Regeneration Technique and Hashirama’s Abstract Notes

Could it be the Sage of Six Paths? He did seem to have been the first person in the Ninja World to learn Sage Mode. More precisely, he was only half human, since half of his bloodline came from the Otsutsuki Clan. Sure enough, the one Sage Slug had mentioned was the Sage of Six Paths. "Yes, Lord Meikyō is from the Uchiha Clan, and the source of the Uchiha Clan's Sharingan was Indra, the Sage of Six Paths' eldest son, who inherited part of the Sage's Eyes' power.