TOKYO—When Keiko Takemiya made her manga debut in 1968, her publisher told her that her original drawings for the comic books could be thrown out. She insisted on getting them back, stamping “Please return this to me” on each page.

More than half a century later, she has a collection of 26,000 drawings from the 180 manga titles she has published, including pioneering works in the genre of manga for girls. The drawings are piled up in her house in southern Japan.