Mahiro and Swim Class (Part 1)

Mahiro and Swim Class (Part 1) (まひろと水泳教室（前編）, Mahiro To Suiei Kyōshitsu (Zenpen)) is Chapter 49 of Onimai: I'm Now Your Sister!. It was originally published on Pixiv on February 23, 2021.

Summary
Mahiro and her classmates begin swim class.

Plot
In the locker room, the girls get ready for swim class. Asahi points out that she wore her swimsuit under her uniform, leading Momiji to ask if that would be uncomfortable.

As Mahiro gets changed, she notes that the dreaded day of swim class has finally come. She is in complete shock at wearing a school swimsuit in front of other people. After Mahiro gets changed and wraps her body in a towel, she encounters Miyo who notes how awful this is. She is worried that they'll see every line and curve, and Mahiro is equally worried about that. Meanwhile Momiji, Asahi, and Nayuta don't have to worry about that due to their lack of development, but still feel left out of the struggle.

Mahiro strips away the towel and puts up her hair in a bun, noting that she feels like yet another layer of her dignity as a man has been stripped away. She also describes this a casual entrapment, being so close to so many girls in swimsuits. Momiji asks Mahiro if she's okay, and asks if it's a little chilly. Mahiro snaps back to reality and agrees with her.

Just then, a cool breeze washes over everyone and Momiji shrieks from the cold. She starts hugging Mahiro and says that she's freezing, much to Mahiro's shock. Mahiro pushes her off and tells her that there is no huddling up. Momiji asks why, but Mahiro doesn't answer. She instead tells Momiji to hug Asahi who is more than happy to do it. Asahi hugs Momiji, and the two feel better all snuggled up. Mahiro sighs with relief, noting that if there was any time to abandon her male instincts, it's now.

Rin claps her hands and tells the girls to gather around. She explains that the pool is very dangerous so they must follow her every instruction. She tells them that if they feel sick or injured to tell her immediately. She also adds that the boys will be having their own lessons on the other side of the pool, and that she expects the girls not to spend their time peeping at the boys.

Miyako notes that it'll be more like the exact oppiste, and Mai promises that they won't do the peeping. Mai then whispers to Miyako that she might peep if even a single boy had abs to speak of, which Miyako agrees with. Mahiro laughs, thinking to herself that it's a fact of life that boys will peep and notes that it's tough being a girl. Mahiro then realizes that she is on the creeped out side this time, and thinks that boys are such brutes, and monsters of lust. Momiji is confused at Mahiro losing her composure.

Mahiro tells herself to calm down and think and remember what being a boy was like, as she knows what is going on through their heads. She notes that it's no skin off he rback, and decides to let those excitement starved boys have their bit of harmless fun.

On the other side, Yuta and Minato see Mahiro sitting in a very cute position and start to get aroused. Satsuki notices them looking, and Minori calls them the worst. Mahiro notes that taking an interest is only healthy. Rin tells the two to settle down and announces that it is time for warm up exercises, something that Mahiro fears.

Mahiro and Miyo pair up for their stretching routine, causing Mahiro to take it back and silently beg the boys not to look at her as all their assets are displayed for everyone to see. Miyo feels the same way, the two being partners in adversity. Momiji notes that being so developed comes with it's own problems, and Asahi agrees. Meanwhile, Yuta and Minato sit in ecstasy and lean forward at the sight of Mahiro.

When it is time to get into the pool, Mahiro dips her foot in and then pulls it out, refusing to get in by claiming that the water is freezing. Asahi says that it's not so bad once you're in, and Momiji has Mahiro get used to it bit by bit by splashing some of it on her. Miyo just tells Mahiro to breathe in and breathe out.

Mahiro eventually gets in and sighs, noting that getting out will be even worse. Momiji notes that that is not a fun thought. She then asks Mahiro if she's any good at swimming, with Asahi noting that they are splitting them up based on ability. Mahiro wonders about it, remembering that she used to swim okay, but decides that she may as well take it easy. She is given a kickboard and put into the beginner group with Miyo, Nayuta, Nemu, and Aoi.

Mahiro notes that this is the life, easy peasy. Nayuta suddenly slips under the water, and Mahiro brings her up. Nayuta is thankful that Mahiro saved her. Mahiro wonders how that happened even though Nayuta had a kickboard. Nayuta cringes, and thinks to herself if only she had the proper buoyancy. She stares at Miyo, believing that having large breasts somehow helps one float better. Miyo realizes that Nayuta is staring at her and covers up her chest. Mahiro wonders if there is any truth to that statement, but Miyo claims that her bust does not help her float.

Rin tells them to start by holding onto the ledge and learning how to kick, which Mahiro agrees to. The girls begin, but Mahiro stops after a few seconds noting that it's really tiring due to a lack of stamina on her part. Rin tells Mahiro that she's sinking and tells her to keep at it. Mahiro does keep at it, but notes that her muscles are going to be sore tomorrow. As Mahiro continues, she notes that this is too much, as her whole lower body feels like death. She cringes over all this motion and her thighs rubbing together.

Mahiro then feels a twinge much to her confusion, and stops to inspect it much to Rin's confusion. Mahiro gropes something between her legs and realizes in horror that her "comrade" has returned, while she's in a swimsuit, in a pool, with a bunch of real girls in swimsuits.

It is revealed that Nayuta somehow launched her kickboard out of the water due to its buoyancy, much to Mahiro's shock.

Characters

 * Mahiro Oyama
 * Momiji Hozuki
 * Asahi Ōka
 * Miyo Murosaki
 * Nayuta Tenkawa
 * Rin Minakata
 * Aoi Tachikawa
 * Nodoka Fushimi
 * Mai Yasaka
 * Miyako Nagase
 * Satsuki Tadaka
 * Minori Shiina
 * Yuta Sakurada
 * Minato Senkawa
 * Nemu Fujimi

Trivia

 * Swim class begins in this chapter, having been previously mentioned as coming up soon by Miyo in "Mahiro and the Seasonal Attire". Ironically, the one thing that Mahiro feared most would happen in that chapter almost does happen here, almost exactly as she imagined it.
 * This chapter marks the first time that Nayuta almost drowns, and at the same time reveals her theory that having a larger bust makes one swim better.
 * Aoi appears in the beginner's group, but her hair was drawn the wrong color. It would be later confirmed to be her in "Mahiro and Swim Class (Part 2)"
 * This marks the third time that Mahiro's "comrade" has regrown. The first time was in "Mahiro's Panic! At the Hot Spring (Part 1)" when the drug wore off for the first time. The second time was in "Mahiro and the Unexpected Guests" when she got caught a cold, causing the drug to temporarily wear off.