Ichiro Sennin

The Ichirō Sennin (仙人 一浪, Sennin Ichirō, lit. Sage of One Way), real name unknown, was a legendary, historical sage who introduced the concept of Reiatsu control to the spiritual world, helping to form what is known today as the Shinigami and is the founder of Soul Society. He is revered as the first Shinigami and is considered one of the most powerful beings to have ever existed, second only to the Soul King.

Background
Long before the Soul Society had come together as a singular concept, deceased souls of the Human World simply wandered through what was then known as Purgatory