Koji Matsumoto is a Japanese singer, songwriter, and actor who is best known for writing the songs on the Captain Underpants album the Super Silly CD o' Fun. He has a YouTube channel called KojiUnderpants, where the music videos of his songs can be found. As of July 2020, this channel has 340 subscribers. He is also included on several live-action digital flip-o-ramas on the Captain Underpants website. It is unknown where he is now, as he hasn't uploaded a new video in 10 years.

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