My Stepmom's Daughter Is My Ex

My Stepmom's Daughter Is My Ex (継母の連れ子が元カノだった) is a Japanese romantic comedy light novel series written by Kyōsuke Kamishiro and illustrated by Takayaki. The series began serialization on the online novel website Kakuyomu in August 2017, with a print version beginning publication under Kadokawa Shoten's Kadokawa Sneaker Bunko imprint in December 2018. The light novel is licensed digitally in North America by J-Novel Club. A manga adaptation by Rei Kusakabe began serialization online by Fujimi Shobo via Niconico Seiga website as part of the Dra Dra Sharp and Dra Dra Flat brands in April 2019. An anime television series adaptation produced by Project No.9 aired from July to September 2022.

Synopsis
When they were middle schoolers, Mizuto and Yume were a completely normal couple. Between flirtations and shouting matches for minor reasons, the two students stayed together until they entered high school, where they finally decided to break up. What they do not know is that fate sometimes has many surprises in store, and that they will be reunited again as stepbrother and sister only after two weeks. Indeed, their respective parents are about to remarry and the former couple will have to live under the same roof on a daily basis. In order not to hinder their parents' happiness, they accept the situation and decide to put in place the rule of brothers and sisters, according to which the first who feels attraction towards the other loses.

Characters

 * Mizuto Irido (伊理戸 水斗)
 * Mizuto is Yume's ex-boyfriend who became her stepbrother when his father, Mineaki, remarried her mother, Yuni. He shares a fondness for reading with Yume and has high grades, leading to a competitive relationship with her. He and Yume were born on the same day. His mother died when he was young.
 * Mizuto is Yume's ex-boyfriend who became her stepbrother when his father, Mineaki, remarried her mother, Yuni. He shares a fondness for reading with Yume and has high grades, leading to a competitive relationship with her. He and Yume were born on the same day. His mother died when he was young.


 * Yume Irido (伊理戸 結女)
 * Her name was Yume Ayai (綾井 結女) before her mother, Yuni, married Mizuto's father, Mineaki Irido, two weeks after she and Mizuto broke up. She and Mizuto were born on the same day. She is an excellent student and is fond of reading novels. She wore glasses when she was in middle school and dating Mizuto.
 * Her name was Yume Ayai (綾井 結女) before her mother, Yuni, married Mizuto's father, Mineaki Irido, two weeks after she and Mizuto broke up. She and Mizuto were born on the same day. She is an excellent student and is fond of reading novels. She wore glasses when she was in middle school and dating Mizuto.


 * Akatsuki Minami (南 暁月)
 * Kogure's childhood friend. She and Kogure previously dated, but they broke up after he developed an illness as a result of her excessive affection for him and had to be hospitalized.
 * Kogure's childhood friend. She and Kogure previously dated, but they broke up after he developed an illness as a result of her excessive affection for him and had to be hospitalized.


 * Kogure Kawanami (川波 小暮)
 * Akatsuki's childhood friend. He and Akatsuki previously dated.
 * Akatsuki's childhood friend. He and Akatsuki previously dated.


 * Isana Higashira (東頭 いさな)
 * Mizuto and Yume's classmate. She is fond of reading books such as light novels. She frequents the library, where she meets up with Mizuto.
 * Mizuto and Yume's classmate. She is fond of reading books such as light novels. She frequents the library, where she meets up with Mizuto.


 * Mineaki Irido (伊理戸 峰秋)
 * Mizuto's father who married Yume's mother. His first wife died while giving birth to Mizuto.
 * Mizuto's father who married Yume's mother. His first wife died while giving birth to Mizuto.


 * Yuni Irido (伊理戸 由仁)
 * Yume's mother who married Mizuto's father after divorcing her first husband.
 * Yume's mother who married Mizuto's father after divorcing her first husband.


 * Madoka Tanesato (種里 円香)


 * Chikuma Tanesato (種里 竹馬)

Production
The first names of Yume and Mizuto come from "oil" (the pronunciation of oil (油) is yu in Japanese) and "water" (mizu means water (水) in Japanese), respectively. The surnames of Irido, Ayai, Higashira (lit. "east head") and Tanesato come from Conan Doyle ("Irido" is almost an anagram of "Doiru", which is the Japanese pronunciation of Doyle), Yukito Ayatsuji (only the word Aya (綾) is taken), Nisio Isin (while Nisio is the surname, it literally means "west tail", the opposite of "east head") and Ellery Queen (while Queen is a duo whose surnames are Dannay and Lee, "Danna-" is pronounced like tane and "Lee" is pronounced like ri ("里" can be pronounced as sato and ri in Japanese)), respectively.

The author Kyōsuke Kamishiro set Yume and Mizuto's birthday as November 3 since both 11 and 3 are prime numbers, which cannot be "separated" (in Japanese, "割る" means "to divide" and "to separate").

The location setting of the story had been kept ambiguous since its web serialization. However, after several years, Kamishiro decided to set it in an actual location, Kyoto, since the story in Volume 8 takes place in Kobe.

Light novels
The series began serialization on the online novel website Kakuyomu on August 7, 2017, with a print version beginning publication under Kadokawa Shoten's Kadokawa Sneaker Bunko imprint on December 1, 2018; the series has eleven volumes as of December 1, 2023. The light novel is licensed digitally in North America by J-Novel Club.

In the Blu-ray set of the anime series, an original what-if story titled Mamahaha no Tsurego ga Imakano datta (継母の連れ子が今カノだった) written by Kyōsuke Kamishiro is included. The story is about what if Yume and Mizuto did not break up before they became stepsiblings.

Manga
A manga adaptation by Rei Kusakabe began serialization online via Niconico Seiga website as part of the Dra Dra Sharp and Dra Dra Flat brands on April 26, 2019, and has been compiled into seven volumes by Fujimi Shobo as of January 9, 2024. At Anime Expo 2023, Udon Entertainment announced that they licensed the manga for English publication.

Anime
An anime adaptation was announced on July 21, 2021. It was later revealed to be a television series produced by Project No.9. The series was directed by Shinsuke Yanagi, with Deko Akao handling the scripts, Katsuyuki Sato designing the characters, and Hiromi Mizutani composing the music. It aired from July 6 to September 21, 2022, on AT-X, Tokyo MX, BS NTV, MBS, and BS Fuji. The opening theme song is "Deneb to Spica" (デネブとスピカ) by Dialogue+, while the ending theme song is "Futari Pinocchio" (ふたりピノキオ) by Harmoe. Crunchyroll has licensed the series.