Shi tai-san (死体さん, 'Mr. Corpse', pronounced more like "shee-tai" in regular speech) is a character found in Madotsuki's dreams.
He is found on The Road section of Dense Woods B surrounded by traffic cones. Interacting with him gives you the Stoplight effect. It symbolizes a traffic incident due to reckless driving.
Using the Cat effect near him causes him to twitch.
