Born in 1619 as the third son of Sen Sotan, with the go name Koshin. When his father Sotan retired in his later years, Sosa inherited the Fushin'an — the Sen family's main tea room — around 1656 and became the first head of Omotesenke. Serving as tea master to the Kishu Tokugawa family, he established ties with daimyo houses and consolidated the iemoto system. He faithfully inherited his father Sotan's spirit of austere wabi while also diligently spreading tea among the warrior class. The name "Omotesenke" derives from the Fushin'an located on the "front" (omote) side of the Sen property. He established a calm, dignified temae, and as the school upholding the orthodox wabi-cha tradition since Rikyu, Omotesenke continues to exert great influence in the tea world to this day. He died in 1672 at age 54.

A gentle man who inherited his father's spirit of austerity while also possessing social awareness. He balanced formality with the essence of Rikyu's teachings.

Omotesenke continues to promote tea as one of the three Sen houses today. Fushin-an is considered one of the most prestigious tea rooms, tracing its lineage directly to Rikyu.