A member of the Gung-Ho Guns.
His saxophone converts sound waves into pain, and can change into a gun. He used his saxophone gun on himself (Trigun Episode 24) in front of Vash The Stampede in the anime. Probably to cause more mental/emotional damage to Vash.
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In the manga, (Trigun Manga Volume 5) he is killed by Legato Bluesummers. "He dislikes both Legato and Knives, witch leads him to try and kill Legato, though he is quickly killed himself. Legato uses Midvalley's body to attack Hoppered. Atferwords, he (his body) is buried, with his saxopone on his gravestone." - Trigun Manga.