Short answers, pulled from the story.
Mitsuyo Seo was a Japanese animator, screenwriter, and director who played a central role in the development of anime. He directed Japan's first feature-length animated film, Momotarō: Umi no Shinpei (1945), and made Ari-chan (1941), the first Japanese work to fully use the multiplane camera.
Momotarō: Umi no Shinpei is a 74-minute wartime propaganda animated film directed by Seo in 1945 and produced for Shōchiku. It is recognized as Japan's first real feature-length animated film, distinguishing it from its predecessor Momotarō no Umiwashi, which ran only 37 minutes.
Seo was a member of the Proletarian Film League of Japan and contributed to leftist animated films before the war. In 1931 he was arrested for his political activities, tortured, and spent 21 days in jail.
Osamu Tezuka, later known as the father of Japanese manga, saw Momotarō: Umi no Shinpei as a teenager and said he was so impressed that he wanted to become an animator for a time. Seo's feature film directly inspired Tezuka during his formative years.
Seo's postwar film Ōsama no Shippo (1949) was rejected by Tōhō as too politically leftist, leaving it without a distributor. Nihon Manga Eigasha, the studio that made it, went bankrupt, and Seo found conditions for animation in the immediate postwar period too difficult, so he left the industry and became a children's book illustrator.
Mitsuyo Seo died on the 24th of August 2010 at the age of 98.