Mihari's Skipped Grades

Mihari's Skipped Grades (みはりと飛び級, Mihari To Tobikyū) is a bonus chapter of Onimai: I'm Now Your Sister!. It was published exclusively in Volume 4 of the manga which was released on July 27, 2020 in Japan, and January 25, 2022 in North America.

Summary
Mihari remembers when she started high school with Kaede.

Plot
Several years ago, Mihari began her first day of high school. While at home getting ready for the day, she looked at herself in her new school uniform and felt great that she was was finally a high school student. She tried to get Mahiro to see her in her new uniform, but only got silence from his room. At this point, Mahiro had begun to isolate himself in his room due to feeling inferior to Mihari and had a "KEEP OUT!" sign taped to the door. Mihari noted that Mahiro was spending more and more time up in his room.

As Mihari walked to school, somewhat disappointed, Kaede, who was sporting a new hairstyle, snuck up behind her and surprised her by placing her hand on her shoulder and calling out her name. Kaede greeted her, but Mihari hid behind a pole asking Kaede who she was, not recognizing her due to her new look. Kaede was somewhat annoyed at Mihari and pointed out who she was. Mihari was surprised, asking Kaede if she was a bad girl now. Kaede got embarrassed at this statement and claimed that Mihari was just biased against pretty girls.

As the two continued to walk to school, Kaede explains that she just decided that she wanted to reinvent herself when she made her high school debut. Mihari notes that that makes sense as Kaede was always into fashion and stuff. This causes Mihari to wonder if people can really make such big changes in their lives.

Kaede notices that Mihari has been gloomy all morning and tickles her from behind, causing Mihari to laugh and tell her to stop. When Kaede finally does, Mihari asks her if she had to make her personality so free-wheeling as well.

Kaede says that people are pretty simple when you get right down to it. She says that just something as simple as putting on makeup can improve your mood, and as you improve your appearance you'll gain confidence. This in turn makes you look even more beautiful. She suggests to Mihari that she should get into fashion as well. Mihari passes on that suggestion, but thinks to herself that improving one's appearance isn't a bad idea. She thinks that she could start by giving Mahiro some fashionably sharp outfits, but decides that if there's no progression than there's no point. Mihari decides that she'll have to do it by force, the sooner the better, and begins formulating a plan in her head. Kaede sees Mihari lost in thought and calls out to her to no avail.

Back in the present, Kaede remarks that shortly after that, Mihari decided to take advanced placement exams and skip ahead to college. She tells Mihari that she's always been smart, but wonders what made her want to go that far. Mihari just laughs at that statement.

Kaede says that she was really lonely after Mihari left, and Mihari apologizes. Kaede asks Mihari what field of research she ended up in anyway, and Mihari chuckles, saying that she's not telling. All the while, she looks at the fruit of her research, Mahiro, who is napping on the couch and smiling.

Characters

 * Mahiro Oyama
 * Mihari Oyama
 * Kaede Hozuki

Trivia

 * This chapter takes place a few years (the specific number unspecified) before the events of "Mahiro's Confusing New Body".
 * This chapter showcases the origins of when Mihari decided to start helping Mahiro reintegrate into society, essentially the birth of her "Onii-chan Rehabilitation Project".
 * The idea to turn Mahiro into a girl came from Kaede's suggestion that using makeup and wearing fashionable outfits causes someone to gain more confidence and feel better about themselves, which could be easier with girl's clothing.
 * The idea to de-age Mahiro came from Mihari realizing that progression was important, and that if Mahiro had a chance to start over he could regrow into a better person.