Yoshiyuki Sadamoto designed the character to meet the staff’s request to include a being that could serve as the series mascot. Because the show is set in Hakone, a town known for its hot springs, Pen Pen was at first conceived as a monkey, but the idea was considered unattractive and was discarded. Pen-Pen is possibly inspired by Mister Pen-Pen (Mr．ペンペン), a genetically modified penguin that appears in a 1986 anime of the same name.

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