Yasu, whose real name is Sayo Yasuda, was the child born from the incestuous relationship between Ushiromiya Kinzou and his daughter Ushiromiya Beatrice. Kinzo offered him to Ushiromiya Natsuhi to raise as her own child, but refused his offer and shoved the servant carrying Yasu off of a cliff, gravely injuring him. Even though Yasu survived, his sexual organs were damaged beyond surgical repair, and was raised as a female within the Fukuin House on Rokkenjima.

He adopts many other personas that appear throughout the series, originating as imaginary friends to cope with the loneliness of being raised in the Fukuin House. Shannon, Kanon, and Beatrice are the result of this, and each personality went on to live as separate figures who fell in love with George, Jessica, and Battler, respectively. Most furniture that appear in Umineko were created this way as well; Virgilia and Ronove were respectively based on Kumasawa Chiyo and Ronoue Genji, his parental figures, and the Seven Stakes of Purgatory were based on the Fukuin household servants he worked with that picked on him mercilessly.

Due to being the last surviving child of Kinzo, he turns out to be the true heir of the Ushiromiya and inheritor of the gold promised for solving the epitaph. He is also behind the forgeries that are based off of the murders on Rokkenjima, and therefore Umineko's framing device. He later kills himself in Requiem of the Golden Witch after surviving the murders and escaping with Battler.

In an "ideal" existence in EP7, he is accepted by Natsuhi and later grows up to be Ushiromiya Lion. In EP5, he is known as "The Man from 19 Years Ago", a form of Sayo whose resentment towards his family took over all aspects of life. As this persona, Battler passed on harassing phonecalls to Natsuhi, attempting to implicate her as the killer on Rokkenjima, from a script which Sayo wrote.

The name "Yasu" is derived the infamous "Yasu was the culprit" meme from Portopia Renzoku Satsujin Jiken (The Portopia Serial Murder Case); in which it's revealed that the killer is Yasuhiro Mano, the player character's assistant who speaks and acts for him.