Retsu is a character from the original Street Fighter game.
He is not Dan Hibiki's sensei, but, instead he is an excommunicated monk & Ryu's sparring partner in Japan.
He, along with the Ninja Geki are the two fighters representing Japan, if the player does not choose another country as their first arena.
Retsu fights using Shorinji Kenpo techniques in front of a temple.
Although he is defeated by the player, he makes another appearance in the manga Sakura Ganbaru, fighting Sakura Kasugano.
His only other appearances have been various Capcom artworks.
Retsu means Strong or Violent in Japanese.