Mahiro's Beach Fun (Part 1)

Mahiro's Beach Fun (Part 1) (まひろの海遊び（前編）, Mahiro No Umi Asobi (Zenpen)) is Chapter 57 of Onimai: I'm Now Your Sister!. It was originally published on Pixiv on October 30, 2021.

Summary
Mahiro and her friends head to the beach.

Plot
Mahiro and her friends arrive at the beach, having previously chosen it as their place to go for summer vacation. Momiji and Asahi are extremely excited. Mahiro slowly peels off her clothes, revealing the polka-dotted two-piece swimsuit with a skirted bottom that Kaede bought for her. She thinks to herself how she can't believe that she's outdoors once again, and in a swimsuit to boot. Kaede calls out to everyone to help them get everything set up. Seeing Kaede's large bust in a bikini top puts Mahiro at ease, and she makes a prayer sign thinking how she loves the beach.

Mihari tells everyone where to put their bags and everyone puts them where she says. Mahiro becomes a bit tired from lugging her bag all the way there. She tells Mihari that she feels a little uneasy in this two-piece, but Mihari can't believe that she's saying that now since they're already at the beach. Mahiro explains that it is better than a school swimsuit, but that there is not much protection where she needs it, flipping up the front of the skirt to prove it. Mihari understands after seeing it, but tells Mahiro not to worry as she has more of the medicine on hand. She takes out a bottle of it and explains that this is a new formulation, and that the ingredients have been adjusted a bit.

Mahiro is intrigued, but before Mihari can explain further, Miyo shows off her new bikini. Asahi is shocked that Miyo is really packing a punch today, and Miyo explains that she decided to be more daring. She is still a bit embarrassed, while Momiji is left in shock. Mihari also becomes shocked at how developed Miyo is and drops the bottle of medicine. Mihari feels gloomy that she's losing to a junior high girl, and is unaware that she dropped the medicine into Nayuta's bag.

Kaede announces that everything is set up, causing everyone to cheer. She tells them to have fun, leading Asahi to ask Kaede what she'll be doing. Kaede says that she is going to sunbathe, and tells Momiji to oil her up, with Momiji agreeing to do so. As Momiji rubs oil on Kaede's back, Mahiro and Miyo get aroused at the sight, with Miyo remarking how mature it looks. Mihari gets competitive and remarks the maybe she'll sunbathe as well. When Mahiro has no reaction, Mihari hides under beach towel and cries that this is no fair. Mahiro giggles and tells everyone to run away, which they do.

In the water, Momiji and Asahi see a set of marker buoys. Asahi suggests to Momiji that they race to one of the buoys, and Momiji tells Asahi that she's on. As the girls start to race, Mahiro notes that they are always really competitive. Miyo grabs an inflatable beach ball and suggests that they do something more laid back.

Miyo prepares to serve the ball and eventually does do. When she serves it, Mahiro becomes amazed by seeing Miyo's bust bounce, calling it a "three balls super move". This distracts Mahiro and causes her to get hit in the face with the ball, which knocks her into the water. Miyo asks Mahiro is she's okay and Mahiro gets up insisting that she's fine. Mahiro then realizes that her bikini top fell off and hides herself in the water. Miyo tells Nayuta to help her look for it quickly.

Momiji ultimately loses the race against Asahi, much to her disappointment, with Asahi smiling and making a hand sign for victory. Mahiro's lost bikini top floats out to where they are and Momiji finds it. Picking it up, she is shocked at how large it is and how developed Mahiro must be.

Once Mahiro's bikini top is returned safely, she decides that the ocean isn't safe and suggests to Momiji and Nayuta that they stay on the beach. Nayuta fully agrees, with Miyo remembering that Nayuta can't swim. The three girls make a sand sculpture of a smiling creature.

Momiji comes over and announces that she borrowed a float. Asahi comes over with a life preserver and says that with it they can all play in the water together. Mahiro is excited about the float, which is in the shape of a banana, and gets on it. Momiji joins her and the two girls hold each other and giggle while floating through the water. Mahiro smiles and remarks that this is the life, while Miyo smiles at the obvious Yuri appeal of this scene.

Nayuta gets into the life preserver, and is excited that she won't sink if she has it. Asahi tells her that that is good and tries to teach her to swim by telling her to kick her legs and swim towards her. Nayuta does so, but is eventually caught up a large wave which flips her upside down under the water, much to Asahi's horror.

After Asahi saves Nayuta from drowning, Miyo asks her if she's okay. Nayuta says that she is all tired out now, while Asahi remarks that she is just glad that Nayuta is safe. Mahiro suggests that Nayuta rest at the base that Kaede set up, with Momiji asking if Nayuta wants them to come with her. Nayuta tells them that they don't need to trouble themselves, and runs off saying that she's just get a cold drink and come back.

Momiji and Miyo admit that as long as Nayuta can still, run they are both glad. Asahi announces that it is time for more swimming, and Mahiro notes that Asahi is sure full of energy.

When Nayuta returns to the base, she sees Kaede asleep on a chair under the umbrella and Mihari asleep on the ground beneath her towel. She decides that she'd better not wake them and heads to her bag. While rummaging through it, she finds the bottle of medicine that Mihari accidently dropped in there earlier and wonders what it is. She thinks that it is an energy shot, but wonders what it was doing in her bag. Remembering that Chitose is always drinking energy shots to stay up late doing her research, Nayuta assumes that she probably snuck one in the bag for her.

Nayuta opens the seal and drinks the medicine in one gulp. She starts to feel strange, and realizes in horror that this isn't an energy shot after seeing the transgender symbol on the outside of the bottle.

It is revealed that Asahi found a crab on the beach and is playing with it by mimicking the crabs claws by making scissor hand signs.

Characters

 * Mahiro Oyama
 * Mihari Oyama
 * Kaede Hozuki
 * Momiji Hozuki
 * Asahi Ōka
 * Miyo Murosaki
 * Nayuta Tenkawa
 * Chitose Azuma

Trivia

 * The events of this chapter are a direct follow up to the events of "Mahiro and the Strategy Meeting" where it was decided that Mahiro and her friends would go to the beach, with Kaede and Mihari joining them.
 * It is revealed in this chapter that Kaede picked out a dark polka-dotted two piece swimsuit with a skirted bottom for Mahiro in "Mahiro and the Strategy Meeting". It is also revealed that Miyo went with the more daring option that Kaede suggested.