Onimai: I’m Now Your Sister!, a.k.a. Onii-chan wa Oshimai! or Big Brother is Done For!, is a seinen gender bender comedy anime series. It features an adult male otaku who gets turned into a little girl.  

Table of Contents

  1. Plot
  2. Onimai Setting
  3. Characters/Voice Actors
  4. Trailer
  5. Where to Watch

Onimai Plot

Onimai at Crunchyroll - Nekotofu/ Ichijinsha/ Onimai Production Committee

At the beginning of the series, Mahiro Oyama lives with his younger sister, Mihari Oyama. Mahiro is a NEET (Not in Education, Employment, or Training) who spends all day at home.

On the other hand, Mihari is a scientific genius who skipped several grades to enter college. She created the drug which turned Mahiro into a girl. 

The drug is not permanent — Mahiro will eventually change back to male. In the meantime, he must adjust to life as a young girl. 

The now “older” sister Mihari treats this as both a scientific experiment, and an opportunity to play with the cute younger sister she never had. She also hopes to reform her brother into a better person, by giving him a second chance to relive childhood.

Naturally, there are complications along the way. After he enrolls in middle school, Mahiro now has classmates who don’t know “her” secret. The girls meet a fun, if somewhat strange, new friend to hang out with. Meanwhile, the boys misinterpret or are weirded out by Mahiro’s boyish attitude.

Although the series talks about such sensitive topics as “that time of the month,” and underwear, it manages to do so without going too far. Onimai does skirt around the edges of fanservice. Mahiro is understandably uncomfortable about changing clothes around other girls, for example.

But despite this, Onimai aims to be a cute, and mostly wholesome, slice-of-life schoolgirl series. It just happens to have a gender-swapped protagonist.

Onimai Setting

Onimai at Crunchyroll - Nekotofu/ Ichijinsha/ Onimai Production Committee

Unlike some shows which take place in Ikebukuro, Shikoku, or Shimokitazawa, Onimai takes place in a generic, fictional modern-day Japanese town. Schools, homes, stores, shopping districts, and the shrine they visit for New Years could be anywhere in Japan.

Mihari is a scientist at the fictional Yotsuba University. The middle school that Mahiro goes to is affiliated with Yotsuba University. 

In addition to the slice-of-life hijinks at school and around town, they also take a trip to an onsen (hot springs) resort in the snowy mountains. Although the location is anonymous, some fans have suggested that it looks a lot like Ginzan Onsen in Yamagata Prefecture.

The main town’s name isn’t mentioned in the anime until the end of episode 12, where it gets a very brief, but potentially revealing mention.

The main characters are riding  a train home from Ginzan, and the train announces that “Onimai” is the next station. However, the kanji used to write “Onimai” can be translated to suggest that there will be a second season. Also, they are sitting in train car 2.

Onimai Characters/Voice Actors

Mahiro Oyama — voiced by Marika Kono

Mahiro at school - Onimai at Crunchyroll - Nekotofu/ Ichijinsha/ Onimai Production Committee

Mahiro is a hikikomori (“shut-in”) otaku who gets transformed into a girl. This happens just before the anime series starts, so we never see exactly what he looked like as an adult prior to his transformation.

As a man, Mahiro loved video games, and manga, especially erotic manga. After his transformation, she still loves video games, which she tries to share (the non-erotic ones) with her new friends.

As a girl, Mahiro is cute, but initially confused about her new body. She’s easily embarrassed by a lot of the feminine stuff that she gets exposed to.

Over time, Mahiro slowly gets used to her transformation. She begins to act less like an otaku, and more feminine, which Mihari considers a positive sign of success.

Noticeably, Mihari continues to call him onii-chan, or “big brother,” when the two of them are alone. 

  • Marika Kono voiced Hanabi in Scorching Ping Pong Girls, Mimi in Digimon Adventure (2020 reboot), and Hinagiku in In the Heart of Kunoichi Tsubaki, among others.

Mihari Oyama — voiced by Kaori Ishihara

Mihari - Onimai at Crunchyroll - Nekotofu/ Ichijinsha/ Onimai Production Committee

Mihari is Mahiro’s younger sister. She’s a lot smarter and more mature than her older brother, with a flashback suggesting that he lived in her shadow even as a child. Before the start of the series, she skips several grades and enters college.

As a scientific genius and researcher, she’s the one responsible for Mahiro’s transformation into a little girl. She treats the transformation like a science experiment. But, she’s also excited at the opportunity to dress Mahiro up and play with her.

Ironically, although she approves of Mahiro’s new appearance, Mihari herself doesn’t dress in a typical feminine fashion.

She cares greatly about her brother, and seems to be largely motivated by the desire to see Mahiro improve.

  • Prior to Onimai, Kaori Ishihara was Lily in Miss Shachiku and the Little Baby Ghost, Shouta in Miss Kobayashi’s Dragon Maid, and Aladdin in Magi: The Labyrinth of Magic, among other roles.

Kaede Hozuki — voiced by Hisako Kanemoto

Kaede - Onimai at Crunchyroll - Nekotofu/ Ichijinsha/ Onimai Production Committee

Kaede is Mihari’s best friend and former classmate. She is a gyaru, who teaches Mahiro about fashion and makeup. 

Kaede also acts like a big sister toward Mahiro and the other middle schoolers. She is literally Momiji’s big sister.

  • Hisako Kanemoto has voiced Ika Musume in Ika Musume, Sailor Mercury in Sailor Moon Crystal, Hinano in Assassination Classroom, and Fushihara-san in Miss Shachiku and the Little Baby Ghost, among others. 

Momiji Hozuki — voiced by Minami Tsuda

Momiji, Onimai at Crunchyroll - Nekotofu/ Ichijinsha/ Onimai Production Committee

Momiji is Mahiro’s best friend at school, and Kaede’s little sister. She is something of a tomboy. She is also athletic and competitive. 

Momiji’s masculine habits initially convinced Mahiro that she was a boy when they first met. 

  • In addition to Momiji, Minami Tsuda voiced Megumi in Wake Up, Girls!, Yui in Yuru Yuri, and Salama in Amagi Brilliant Park, among others.

Asahi Oka — voiced by Kana Yuki

Asahi Oka - Onimai at Crunchyroll - Nekotofu/ Ichijinsha/ Onimai Production Committee

Asahi is one of Mahiro’s friends at school, and part of Mahiro’s foursome (along with Momiji and Miyo). She is energetic and athletic, but sometimes comes across as dumb or naive. She likes to give people nicknames.

  • Kana Yuki also performed Hachiko on Ai Tenchi Muyo!, and the narrator on Bananya and the Curious Bunch, among others.  

Miyo Murosaki — voiced by Natsumi Hioka

Miyo Murosaki - Onimai at Crunchyroll - Nekotofu/ Ichijinsha/ Onimai Production Committee

Miyo is part of Mahiro’s foursome at school.  She’s a fan of yuri, both in manga and in real life. Her interest in the subject is a very poorly kept secret.

She’s also the quiet and shy member of Mahiro’s group.

  • Natsumi Hioka has been Koharu in Slow Loop, Hinaichi in The Vampire Dies in No Time, and Shii Eniwa in Super Cub, among other roles.

Onimai Trailer

Promo video from Toho Animation on YouTube (in Japanese):

Onimai Release Dates

Onimai at Crunchyroll - Nekotofu/ Ichijinsha/ Onimai Production Committee

Manga artist Nekotofu created the original Onii-chan wa Oshimai! manga in 2017. After appearing on the art website Pixiv and elsewhere online, it was picked up by Kodansha in 2019. It currently runs in Kodansha’s Monthly Comic Rex

The first season of the anime series ran from January to March 2023.  So far, a second season has not been announced.

Where to Watch Onimai

Onimai at Crunchyroll

Onimai is available at Crunchyroll in many places. Because of geo-locking, you may need a VPN to watch in some regions.

Medialink’s Ani-One Asia YouTube channel also has the series in Hong Kong. You need to be a member to watch.

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