Blu-ray Volume 1 Appendix Manga

The Blu-ray Volume 1 Appendix Manga is a special chapter of Onimai: I'm Now Your Sister!. It was released exclusively as a part of Volume 1 of the Blu-ray release of the anime series based on the manga which was released on April 19, 2023.

Summary
In this exclusive chapter which takes place in the anime's continuity, Mahiro tries to adjust to life as a girl following the start of Mihari's experiment on her.

Plot
Shocked over having been turned into a middle-school aged girl, Mahiro falls back on her futon and sighs. Mihari tells Mahiro to snap out of the daze already, which caused Mahiro to sit up and angrily remind Mihari who's at fault here.

Mahiro notes that whenever she tries to do anything, she realizes how much her body has changed. Even her hands are so small, which is evident when she runs them along her game controller. Mihari claims to Mahiro that if she just moves around she'll get used to it. She suggests that Mahiro do this by cleaning up her room, leading Mahiro to angrily accus her of shoving morals in at every turn. Mahiro ultimately consents though with a sigh, as she wasn't doing anything else, and Mihari wishes her luck.

Mahiro starts to clean up her manga and begins tossing garbage into the wastebasket. As she reaches for an empty bag of chips and a half-empty cola bottle, she freezes in terror. She realizes that if only she hadn't drank the cola this never would've happened. This leads her to wonder about the events the previous night that led to her turning into a girl.

Normally, Mihari would never shut up about nutritional balance and healthy eating, but last night was different. Mihari called to Mahiro from outside the door that she had brought him a late-night snack consisting of the bag of chips and the cola bottle tainted with the feminization drug. Mahiro didn't answer, so Mihari said that it's nice to have a treat every so often and that she'd leave it in front of his door. Some time later, Mahiro opened his door and took the snack, noticing that Mihari had also left a note reminding Mahiro to brush his teeth aftwerwards.

Back in the present day, Mahiro notes that it was a tad suspect, but that she never thought that this wold happen. Swishing it around, she realizes that there is still a fair bit of it left and decides to flush it.

Mahiro then had a terrifying thought, realizing that the drug worked this well even when she didn't finish it all off. She also notes that it turns back one's clock as well and wonders what would've happened if she had drunk the whole thing. Mahiro had a terrifying image of that scenario, imagining that the drug would've turned her into a baby girl with Mihari deciding to reraise her from the top. The thought leaves Mahiro horrified.

At that moment, Mihari bursts through the door asking Mahiro if she's cleaned her room. Mahiro, still deep in thought, shrieks out and yells for someone to save her. Mihari is confused and asks Mahiro what's wrong.

Trivia
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 * According to Nekotofu, although the plot of the chapter is meant to be canon to the anime as seen by the prescence of the cola bottle, it's events can also be considered for the most part as canon to the manga as well. Thus, he designates the story as being "Chapter 1.25", taking place in between the events of "Mahiro's Confusing New Body" and "Mahiro and the Bathroom". He mentions that in the anime, the events of this chapter thus take place during the events of "Mahiro's Confusing New Body".