Onimai

The series has been serialized online since 2017; it is also currently serialized in Ichijinsha's Monthly Comic Rex magazine since April 2019. It is licensed in English by Kodansha USA. An anime television series adaptation by Studio Bind aired from January to March 2023; it gained high ratings, although it received a mixed reception from foreign critics.

It is recorded as the work of Nekotofu. Publication is recorded by REX Comics and Ichijinsha. The recorded date of publication is 2018. It is associated with Japan. Recorded genres are comedy anime and manga and slice of life anime and manga.

The series follows Mahiro Oyama, an otaku and NEET who lives alone with his scientist younger sister Mihari. One day he wakes up to find himself transformed into a young girl, a result of one of Mihari's experiments. Mahiro must now learn to become comfortable in his new body whilst living life as a middle school girl.

Mahiro Oyama (緒山 真尋, Oyama Mahiro) Voiced by: Marika Kōno The protagonist of the series, Mahiro was previously an otaku man living with his younger sister Mihari until he was turned into a girl as a result of one of her experiments. Mahiro ends up transferring to a middle school. Mahiro is fond of playing video games and mainly stayed at home before being turned into a girl. Mihari Oyama (緒山 みはり, Oyama Mihari) Voiced by: Kaori Ishihara Mahiro's younger sister and a scientist. She turned Mahiro into a girl as a result of one of her experiments. Due to her high intelligence, she skipped several grades and is now a university student. Kaede Hozuki (穂月 かえで, Hozuki Kaede) Voiced by: Hisako Kanemoto Mihari's classmate during middle school and a gyaru. She is Momiji's older sister. She is good at cooking and using cosmetics. She also works part-time at a fast food restaurant. Momiji Hozuki (穂月 もみじ, Hozuki Momiji) Voiced by: Minami Tsuda Kaede's younger sister and Mahiro's classmate. She has a tomboyish personality and greatly admires Mahiro. She has been friends with Asahi for several years. Asahi Ōka (桜花 朝日, Ōka Asahi) Voiced by: Kana Yūki Mahiro's classmate. She has an energetic personality but tends to not perform well in tests. She also calls Mahiro by many nicknames. Miyo Murosaki (室崎 美夜, Murosaki Miyo) Voiced by: Natsumi Hioka Mahiro's classmate. She is interested in yuri and tries to hide it poorly. She enjoys imagining her friends in romantic situations. Miyo is notably bustier than her classmates and embarrassed about it. Nayuta Tenkawa (天川 那由多, Tenkawa Nayuta) Mahiro's classmate who joins during their second year of middle school. She is a bright, yet shy girl who is the ward of Mihari's faculty advisor Chitose Azuma, who is having her monitor Mahiro without Mihari's knowledge. After revealing to Mahiro that she knows the truth about her, she is quickly accepted into Mahiro's group of friends. She is very knowledgeable due to the many hours she spent in the university library, and speaks in a very polite tone with everyone she encounters. Chitose Azuma (吾妻 ちとせ, Azuma Chitose) Voiced by: Mayu Mashiro (drama CD) Mihari's faculty advisor and senpai at the university Mihari attends, as well as Nayuta's guardian. She made key contributions that allowed for the creation of the drug that turned Mahiro into a girl. Although Mihari believes that she did this out of the goodness of her heart, she is actually using Mihari to fulfill an unknown agenda and enrolled Nayuta into the same class as Mahiro to spy on her without Mihari's knowledge.

Prior to creating the series, Onimai's author, Nekotofu, was a fan of the "gender bender" genre of anime and manga that included such works as Ranma ½, Kashimashi: Girl Meets Girl, and Idol Pretender. As such, Nekotofu had harbored a longtime interest in creating his own such series, and was inspired to do so after seeing Your Name. He was also motivated by the positive response to a doujinshi he had created which also featured similar gender-bending elements. Nekotofu began self-publishing Onimai on Pixiv in January 2017, and later on Niconico Seiga, where it became significantly more popular. In April 2019, the manga began serialization in Ichijinsha's Monthly Comic Rex magazine. Nekotofu continues to self-publish the series online and produce doujinshi with permission from Ichijinsha.

Written and illustrated by Nekotofu, Onimai: I'm Now Your Sister! began serialization online via Pixiv and other platforms in 2017. It also started in Ichijinsha's Monthly Comic Rex magazine on April 27, 2019. As of May 2026, eleven tankōbon volumes have been released by Ichijinsha. In North America, Kodansha USA is publishing the series digitally in English.

An anime television series adaptation was announced on April 22, 2022. It is produced by Studio Bind and chief directed by Shingo Fujii, with Michiko Yokote as head writer, character designs handled by Ryo Imamura, and music composed by Daisuke Achiba and Alisa Okehazama. The series aired from January 5 to March 23, 2023, on AT-X and other networks. The opening theme song is "Identeitei Meltdown" (アイデン貞貞メルトダウン) by Enako feat. P Maru-sama, while the ending theme song is "Himegoto*Crisisters" (ひめごと＊クライシスターズ) by Marika Kōno, Kaori Ishihara, Hisako Kanemoto, and Minami Tsuda. Crunchyroll streamed the series. Medialink has licensed the series in Asia-Pacific.

Onimai: I'm Now Your Sister! ranked fifth in Da Vinci's Next Manga Award 2018 in the web manga category in August 2018, and ranked ninth in Web Manga Overall Election 2019 in October 2019. The manga also ranked third in AnimeJapan's Manga We Want to See Animated Ranking in 2020, and ranked seventh in 2021.

In February 2023, fans of Onimai sent numerous gifts to the staff of Studio Bind, the production studio of the anime, which series director Shingo Fujii posted photos of in a tweet voicing thanks for the support.