Sanpei Shirato was the pen name of Noboru Okamoto, a Japanese manga artist born in Tokyo on February 15, 1932, who died on October 8, 2021. He is recognized as a pioneer of the gekiga genre, which emerged as a style of manga intended for adult audiences with more realistic art and serious themes. His background was shaped by his father, the proletarian painter Tōki Okamoto, and the harsh realities of World War II, experiences that instilled in him a critical perspective on social structures.

Shirato began his artistic career as a kamishibai, or picture-card show, artist after finishing middle school. He entered professional manga in 1957, initially creating works in the rental book market. His breakthrough came with the series Ninja Bugeichō, which ran from 1959 to 1962. This historical ninja epic captured the attention of students and intellectuals and is often cited as one of the first examples of gekiga for its violent content and adult themes.

In 1964, Shirato co-founded the influential manga magazine Garo, which became a vital platform for alternative and experimental manga. His series Kamui Den, serialized in Garo from 1964 to 1971, is considered his most important work. The story follows a ninja who rebels against the oppressive feudal system of the Edo period, themes that permeate much of his oeuvre.

Many of his original manga works were adapted into anime. The anime Shōnen Ninja Kaze no Fujimaru, which aired from 1964 to 1965, was based on his manga Kaze no Ishimaru. The 1969 television series Ninpu Kamui Gaiden and its 1971 film adaptation Ninpu Kamui Gaiden: Tsukihigai no Maki were derived from his Kamui Gaiden series. Additionally, his manga Sasuke was adapted into a television anime that ran from 1968 to 1969. The acclaimed film director Nagisa Ōshima adapted his Ninja Bugeichō into the 1967 film Band of Ninja, an unusual work consisting of static manga panels with voiceover.

Shirato's artistic identity is defined by a realistic drawing style and a focus on historical dramas, particularly those featuring ninja. His works serve as a vehicle for social criticism, consistently exploring themes of oppression, discrimination, and class struggle, often drawing parallels between historical feudal Japan and contemporary society. His influence on the manga industry is substantial; he is regarded as a foundational figure of the gekiga movement, and his production company, Akame, which he founded with his brother Tetsuji Okamoto, established new models for manga creation. His legacy has influenced numerous later artists, including Hayao Miyazaki and Masashi Kishimoto.