Mahiro's Big Secret (Part 2)

Mahiro's Big Secret (Part 2) (まひろと大事なヒミツ（後編）, Mahiro To Daiji Na Himitsu (Kōpen)) is Chapter 40 of Onimai: I'm Now Your Sister!. It was originally published on Pixiv on May 24, 2020.

Summary
Nayuta reveals that she knows the truth about Mahiro

Plot
Nayuta has just called Mahiro "Onii-san", causing Mahiro to realize that she knows who Mahiro really is. Mahiro thinks that this could just be a simple misunderstanding and decides to play dumb for now. Mahiro asks Nayuta what she could possibly mean and says that she doesn't get it at all. Nayuta records Mahiro's reaction on her tablet calling it a wonderful performance. She says that she can hardly tell the Mahiro is actually an adult male, causing Mahiro to cringe as she realizes that Nayuta really does know the truth about her.

Mahiro collapses in gloom, thinking that it's all over. She thinks that everyone will arrest her and burn her alive, crying while imagining a newspaper article reading "He might look pretty, but... Invasion of the cross-dressing pants wetter!". Nayuta notices Mahiro's reaction and tells her not to worry, claiming that she's here to help. Mahiro is surprised by this, especially at the idea that Nayuta isn't going to extort her or blackmail her. nayuta rubs Mahiro's hair to comfort her and assure her that she is telling the truth.

Mahiro gets up and asks Nayuta to tell her her story, and Nayuta is amazed that she managed to contain Mahiro's outburst. Nayuta says that her full name is "Nayuta Tenkawa" and claims to be Mihari's research assistant. Mahiro is surprised, but also relieved, to find that Nayuta is from Mihari's lab. Nayuta continues by saying that she's here to make sure Mahiro's peaceful school life proceeds smoothly. Mahiro thanks her a bunch for that.

Nayuta takes out her tablet and begins ordering spare underwear saying that she will make sure that they have replacements ready for the next time this happens. Mahiro apologizes for all this just as the bell rings.

Nayuta realizes that the assembly has ended, and the two hear the students in the hall heading back to class. Mahiro says that this is no good and suggests that they head back to class, saying that they can go over the details another time.

As the two exit the office, they are spotted by Momiji who asks Mahiro why she was in the nurse's office. Mahiro is surprised to see Momiji, Asahi, and Miyo waiting for her and claims that after she went to the restroom she started feeling bad. Nayuta appears next to Mahiro and continues by saying that she happened to be around so she helped to look after Mahiro.

Momiji doesn't know Nayuta, so Nayuta introduces herself by name, saying that she's in the same class as them this year. Momiji understands and thanks Nayuta for helping out Mahiro like that. As Momiji begins to introduce herself, Nayuta brings up intel on all three of them on her tablet and recites their full names, stating that they are exactly as her intel describes them. The three girls are surprised by this, while Mahiro notes that Nayuta's ability to do that is scary.

Asahi tells Nayuta that she doesn't need to be so formal about it and tells her to call her Asahi like everyone does. Asahi adds that if Nayuta is Mahiro's friend, she's their friend too. Asahi tells Nayuta that it is nice to meet her, giving her the nickname "Nayutan", much to Nayuta's surprise.

Mahiro whispers in Nayuta's ear telling her that she can call her "Mahiro", as they are both girls after all. Nayuta says she will, but absentmindedly starts to refer to Mahiro as "Onii-" before realizing it and covering her mouth. Momiji gets confused while Mahiro gets angry at Nayuta for saying that. To cover it, Mahiro claims that the two were playing a (nonexistent) game where you put your index fingers on the top of your head and say "Oni-Oni!". Asahi claims to get it, saying that they were just playing the Oni-Oni game. Momiji is confused, saying that she has never heard of this game before in her life.

Mari calls to them from down the hall, saying that she knows that they're friends, but they need to cut the conversation short and head for their seats. The girls apologize and say that they will, and decide to hurry back to class. As Asahi leaves, she suggests to Nayuta that they should hang out later, leaving Nayuta surprised.

As the others leave, Mahiro notes that she's never been more thankful that Asahi is an idiot and notes that Nayuta is pretty absentminded herself. When Nayuta doesn't answer, Mahiro turns to her wondering what is wrong. Nayuta smiles, noting the she made a friend. Mahiro is confused by her reaction and pull Nayuta by the hand down the hall saying they should go, also to Nayuta's surprise. Mahiro thinks to herself that Nayuta seems like she's got her own problems, and that it's to be expected is she managed to become Mihari's research assistant at her age. Mahiro notes that Nayuta wouldn't have much time to make friends if she's studying so hard.

Mahiro then realizes that it would be next to impossible for someone as young as Nayuta to have a job like that and decides to ask Nayuta if she would happen to be an adult like her. Nayuta just giggles and says that that information is strictly need-to-know. Mahiro just sighs and says "I see".

Later, Mahiro announces her presence as she returns from school. Mihari, who is reading on the couch, asks Mahiro how the new school year is treating her so far. Mahiro says that she should know and asks Mihari why she never tells her these things in advance, saying that she almost had a heart attack. As Mahiro storms upstairs to her room, Mihari wonders what got in Mahiro's bonnet. She wonders what Mahiro meant by "these things", revealing that she has no connection to Nayuta or Nayuta knowing the truth about Mahiro.

It is revealed that Asahi asked Nayuta if she could see her tablet for a second, but Nayuta backed off saying that it's strictly need to know.

Characters

 * Mahiro Oyama
 * Mihari Oyama
 * Momiji Hozuki
 * Asahi Ōka
 * Miyo Murosaki
 * Nayuta Tenkawa
 * Mari Nishinomiya

Trivia
第40话
 * The events of this chapter take place directly after the events of the previous chapter "Mahiro's Big Secret (Part 1)".