ONIMAI: I'm Now Your Sister! (Onii-chan wa Oshimai!, or "Big Brother is Done For!") is a comedic Gender Bender manga by Nekotofu. The manga follows resident Hikikomori Mahiro Oyama, who wakes up to find himself in the body of a young girl. As it turns out, his genius little sister, Mihari, had slipped one of her experiments into his food and turned him into a girl on purpose! While he's uncomfortable with the surprise, Mihari reveals that she's doing it to rehabilitate Mahiro into a more outgoing individual after some untold time living a sedentary lifestyle—and also because she wants a little sister to dote on. Now Mahiro has to navigate his new body as a girl, rediscover the joys of life outside of his bedroom, and deal with all of his sister's crazy antics.
The manga began in 2017 and was originally self-published online for free on Nekotofu's Pixiv account and other platforms, with compilations of chapters sold every few months at doujin events like Comiket; it would later be picked up for serialization in the seinen magazine Monthly Comic Rex starting in 2019. It eventually received an anime adaptation in Winter 2023 by Studio BIND that aired from January to March of 2023.
ONIMAI: I'm Now Your Troper!:
- Academic Athlete: Mihari is a Grade Skipper and a genius who helped develop the sex change drug, and was also a talented runner during her middle school days.
- A-Cup Angst: Several of the girls show different degrees of this, often in relation to Miyo's assets.
- Momiji is painfully aware of her total lack of chest, and often gets jealous of Mahiro and Miyo. It doesn't help that Mahiro keeps carelessly pointing it out.
- Asahi is less conscious about it, but being faced with Miyo's development is one of few things that make her drop her Genki Girl attitude.
- While she is fairly gifted herself, Mihari doesn't take it well when she's out-boobed by a middle-schooler (Miyo again) during a Beach Episode.
- Adaptational Curves: Kaede especially gets hit with this in the anime adaptation. This is best depicted in the scene where the gang goes swimsuit shopping; while Kaede only has a modest figure in the manga, the anime version greatly ups her bust size and hips along with giving her a skimpier swimsuit.
- Attractive Bent-Gender: When Mahiro's class sets up a maid cafe diring a School Festival, boys are forced to wear maid uniforms too. Yuuta and Minato make particularly convincing girls, giving Mahiro a moment of trouble recognizing them and drawing enough attention from the clients to make girls jealous.
- …But He Sounds Handsome: Mahiro once tells Momiji that "big brother" (his male self) went to study in a nice university far away and is doing well living by himself. She seems to believe that, but he clearly feels awkward and guilty for lying.
- Can't Hold Her Liquor: Mihari, of the "gets drunk off amazake" note variety. And off of one tiny sip at that.
- Central Theme: It's very much a story of rehabilitation and getting a second chance at life. The anime takes this to heart and has each episode explore a stepping stone into Mahiro's journey of becoming a girl and understanding self-discovery and self-fulfillment in life.
- Coming of Age Story: The series shows how Mahiro learns to develop better as a new person as although he is reluctant to accept his new gender at first, eventually he becomes ok with it, and learns to appreciate the outside world.
- Compulsory School Age: Invoked when Mahiro is caught at an arcade by a truant officer on a weekday. Though it plays into Mihari's plan to get Mahiro out and socializing more so she goes along with it. Mahiro later enrolls in middle school, playing the trope straight.
- D-Cup Distress: Miyo is no fan of her own rather ample assets.
- Dragged into Drag: During a School Festival event, the boys in Mahiro's class propose a maid cafe. In return, girls decide that boys have to wear maid uniforms too.
- Excuse Plot: The story is largely a comedic rehabilitation narrative for Mahiro, as he learns to slowly shed his hikikomori ways and rebuild a social life. Did Mihari have to turn him into a girl to do it? No, but she wanted to.
- Fake Period Excuse:
- During Mahiro's first PE class, Miyo mistakes his dread at having to run a marathon for menstrual pains, and asks the teacher to let him sit it out. Mahiro doesn't even realize the misunderstanding until later, and Mihari warns him against using this excuse again.
- When the sex change drug starts to wear off in the middle of a swimming lesson, Mahiro tells the teacher that "it started unexpectedly" to get excused and take another dose.
- First Period Panic: An early chapter/episode is devoted to Mahiro getting his first period. While as an adult he's likely to have at least some knowledge about periods, the shock factor and the emotional damage of another reminder that he's a girl now leave him curled up in his bed and sobbing.
- Fright-Induced Bunkmate:
- During an Amnesia Episode, Mahiro's mind regresses to an actual middle school girl and he gets traumatized by one of the porn books in his room. Afraid of staying there, he goes to bed with Mihari, who is all too eager to indulge him.
- During a thunderstorm-induced blackout, Mahiro remembers Mihari's Fear of Thunder and spends the night with her, leading to them sleeping together while she's hugging him.
- Grade Skipper: Mihari and Kaede used to be in the same class when they were younger but Kaede is now currently attending high school while Mihari is at university.
- Harsh Word Impact: Mahiro ends up is on the receiving end of this multiple times for various reasons, including Mihari's Brutal Honesty, guilt over lying about "big brother", or others inadvertently reminding him of his real age.
- Helping Another Save Face:
- When Mahiro doesn't quite make it to a movie theater restroom and wets himself, Kaede helps him clean up and keep it a secret from Mihari. This leads to him seeing her as another sister.
- When Momiji wets her bed during a sleepover at Mahiro's place, he helps her hide it, lets her take his bed, and even takes the blame in the morning. Considering the circumstances and that Momiji and Kaede are sisters, it counts as a Call-Back to the above.
- He Who Must Not Be Seen: While Mahiro's male self occasionally appears in flashbacks, we never see his face or any major features. Later on, he starts appearing with a face as a child, but his adult self stays unseen.
- Hotter and Sexier: While the manga has its share of nudity and raunchy jokes, the anime has much more Fanservice and Male Gaze by comparison, especially in the first episode with how much focus it puts on Mahiro's new body.
- Man, I Feel Like a Woman: After Mahiro gets over the initial panic after his transformation, first thing he tries to do is spending some quality time with his new body and some eroge. Mihari puts a stop to it, telling him that experiencing a girl's orgasm would fry his brain. Even after that, he still examines his new body in a mirror while bathing.
- Minor Living Alone: Although Mihari is a borderline adult (and a university student through Grade Skipping), she has been taking care of both herself and Mahiro for an unknown period of time while their parents are away.
- Mistaken for Flirting: Many of Mahiro's interactions with male classmates end up as this. Usually he's just trying to socialize with "fellow boys", but they inevitably fall for him more and more.
- Mistaken for Prostitute: Due to some unfortunate phrasing, Mahiro assumes that his classmate Miyako engages in Compensated Dating. After learning that her imagined "sugar daddy" is her actual father and she's a total Daddy's Girl, he comes to apologize.
- The Nicknamer: Asahi does this for pretty much everyone. Most get only a slight variation of their name like "Mahiron" and "Momijiro", but Mihari in particular gets called "Professor".
- Odd Friendship: Asahi is a ditzy Genki Girl, while Nayuta is calm, serious and science-minded. They develop a mostly-offscreen close friendship, and other characters are surprised to learn they hit it off so well.
- Parental Abandonment: Mahiro and Mihari's parents are completely absent from the story. A side chapter explains that they are on a long business trip overseas, and their mother regularly exchanges messages with Mihari.
- Playboy Bunny: The anime's ending credits prominently feature Mahiro in a Playboy bunny outfit that the other main characters make him wear.
- Porn Stash:
- Mahiro's is quite extensive and not really a "stash" and more "scattered all over his room" (especially in the anime). This becomes an issue when Momiji comes over to visit the first time.
- Implied with Miyo during a visit to her room: Mahiro notices a conspicuously lily-patterned closet, and guides Asahi away from it when the latter goes snooping around.
- Asahi's brother Yuuya has several hiding places in his room, which Mahiro easily finds (and is impressed by) while looking for books there.
- Real After All: In one chapter, Mahiro and Momiji are stuck in the school building at night and have to run away from a ghost. Eventually they are found by Nayuta and realize it was just her following them... only for the final page to show the real ghost wandering the school.
- Recurring Extra:
- Two minor characters (the lingerie shop lady and the hair salon lady) reappear on several unrelated occasions early in the story (and even at a point much later in the manga), usually commenting on Mahiro's situation or appearance.
- Mahiro's classmate Fujimi Nemu appears often as her seat is right in front of his, but has no lines whatsoever and almost no scenes focusing on her.
- Rhetorical Question Blunder: Mahiro gets one at the start of the manga after being turned into a girl.
- A Rotten Time to Revert: At several points Mahiro's transformation starts reversing, which invariably begins with his penis growing back. When that happens, he's usually in a situation where it's easy to notice (hot spring, swimming pool), or he almost gets into some revealing accident.
- Running Gag:
- The manga has quite a few scenes of the characters rushing to or being in the bathroom. Lampshaded later on, when Mahiro comments it's been a while since his last bathroom trouble long after bathroom scenes became less frequent.
- Two boys from Mahiro's class (named Yuuta and Minato) get repeatedly mistaken for perving on Mahiro or being mean to him, and two girls (Satsuki and Minori) scold and/or smack them for it.
- On many occasions, Mahiro hides behind an object or another person and asks "Who...?" after meeting someone new - or, more often, someone he already knows but with a new look.
- In the anime, several episodes feature background stories with birds.
- Second Law Of Genderbending: While Mahiro is reluctant at first, his sister bringing him out of his shell and forcing him into healthier habits (along with giving him cuter, more flattering clothes) slowly accustoms him to liking this new chapter of his life. He still at times wishes he would turned back into a guy, though.
- Secret-Keeper: Nayuta is the only one of Mahiro's new peers who knows about him being a guy. This is due to her being a (self-proclaimed) assistant to one of the scientists behind the drug.
- Series Mascot: The series features various stuffed toys and other items with a big "C:" smile. They represent the author Nekotoufu, who has such a face on their Twitter avatar.
- Shout-Out:
- When Mahiro checks out his new body in the mirror during his first time bathing with it, he holds out a hand to cover his private parts in a way referencing the "one finger selfie challenge" meme
(link NSFW) that was viral at the time that chapter was released. - In the anime's second episode, Mahiro can be see playing Guilty Gear -STRIVE-.
- A recurring in-universe anime series is called Mahou Kantai☆Love Master, mixing four real franchises together. A few characters visible on its cover resemble main heroines of Puella Magi Madoka Magica.
- In chapter 80, Miyo and Mahiro read a Yuri Genre Visual Novel titled "A Kiss for the White Lily", named after yuri VN series Sono Hanabira ni Kichizuke wo / A Kiss for the Petals and the manga Kiss and White Lily for My Dearest Girl.
- In episode 9, Asahi wears a green jacket and a white mask with a smiley face, closely resembling Dream.
- When Mahiro checks out his new body in the mirror during his first time bathing with it, he holds out a hand to cover his private parts in a way referencing the "one finger selfie challenge" meme
- Teen Genius: Mihari. Not only did she create the means by which Mahiro was changed from an adult man to an adolescent girl she's attending university despite being only high school age. See Grade Skipper above.
- Wanted a Son Instead: Sibling variant. Mihari means well with her experiment on Mahiro, but at times she expresses that she's always wanted a sister and takes every opportunity to indulge in treating Mahiro like one.
