Momiji is the younger sister of Kaede. She is in middle school, and is in the same class that Mahiro enters when they reenter school. Momiji likes to dress, look, and act as a tomboy, and many times she is taken for a young teen/tween boy. However, Momiji gives no indication that she is in any way a transgender tween. She appears to only be gender non-conforming.

However, she can be quite feminine in appearance on select occasions, like when she wears the middle school female uniform, a cute jumper dress, to school. She forms a strong friendship with Mahiro, and helps Mahiro navigate the middle school academic and social challenges of school. 

Many times she comes to Mahiro's defense when Mahiro is bewildered or frightened by somebody or something happening at school, or if it is perceived that the boys are picking on her. She is part of a tight-knit social circle foursome of friends at school, with the other three girls being Mahiro, Asahi, and Miyo-chan.

Mahiro Oyama was just a normal erotic-game loving dude…until he woke up one morning as a woman! Turns out his mad-scientist little sister, Mihari, tried out one of her new experiments on him…with a disastrous outcome, as far as Mahiro’s concerned! But Mihari is as determined to study him as he is determined to go back to his shut-in, game-playing life, and one thing’s for sure…life is going to get a whole lot weirder from here on out! 
 
(Source: Kodansha USA)

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She announced on June 8th, 2024 that she got married to fellow voice actor [Tatsumaru Tachibana](https://anilist.co/staff/137436/Tatsumaru-Tachibana).

Mahiro Oyama was just a normal erotic-game loving dude…until he woke up one morning as a woman! Turns out his mad-scientist little sister, Mihari, tried out one of her new experiments on him…with a disastrous outcome, as far as Mahiro’s concerned! But Mihari is as determined to study him as he is determined to go back to his shut-in, game-playing life, and one thing’s for sure…life is going to get a whole lot weirder from here on out!
 
(Source: Kodansha USA)
 
Note: Includes 57 extra chapters.

An anthology comic of Onii-chan wa Oshimai! by various mangaka.

Specials chapters of Onii-chan wa Oshimai! bundled with the mini dramas CD sold during Comic Market.

Includes: 
1) まひろちゃんお誕生日会場 (2022 March 6) 
2) みはりちゃんお誕生日会場 (2022 April 22) 
3) あさひお誕生日会場 (2022 May 5) 
4) みよちゃんお誕生日会場 (2022 June 18) 
5) かえでちゃんお誕生日会場 (2022 September 12) 
6) もみじお誕生日会場 (2022 October 20) 
7) なゆたんお誕生日会場 (2022 November 23) 
8) まひろちゃんお誕生日会場 2023 (2023 March 6) 
9) みはりちゃんお誕生日会場2023 (2023 April 22)

A booklet bundled with the Onii-chan wa Oshimai! volume 7 special edition.

Special manga drawn for the Blu-Ray release of the anime Onii-chan wa Oshimai!.

Momiji Hozuki