Mahiro Oyama

Mahiro Oyama (緒山 真尋, Oyama Mahiro) is a former otaku slacker and the brother-turned-sister of Mihari Oyama, having been transformed into a middle-school aged girl by one of Mihari's experiments. She is voiced by Marika Kōno in the anime.

Appearance
Mahiro's appearance as a guy directly before the transformation has never been fully seen. As a kid, his face looked surprisingly similar to how it currently is, but with his pinkish-gray hair being much shorter.

After being transformed into a middle-school aged girl by Mihari, Mahiro's hair now stretches to the middle of her back with a single strand on the top poking up and two longer stands that fall to the front and cover her ears. She is also now slightly smaller than Mihari and gives off the appearance of an extremely cute and adorable young girl. Due to Mihari having replaced all her male clothes, Mahiro is now seen wearing skirts and dresses almost exclusively. Despite this, she rejoyces whenever she gets the chance to wear what can be considered boyish-clothing.

As time goes on, Mahiro starts to mature both physically and emotionally. While still cute, her outfits begin to strike a more mature cord through the use of tights and jackets. She grows slightly taller and her bust size also increases allowing her to upgrade to standard bras.

Personality
Mahiro used to be a degenerate slacker with a love only for manga with stories of bondage and sex slavery, and staying home playing video games. It was noted that before the start of the series, he hadn't left his house for two years. According to Mihari, Mahiro wasn't always like this.

When they were younger they were very close and got along famously. Mahiro would buy his sister stickers when she asked for them, won her stuffed animals at amusement park games, and even gave her her signature hair clip as a reward for doing well on a test. He stopped making time for her as he got older and her achievements surpassed anything he could ever hope to achieve, as the result of a combination of an inferiority complex next to Mihari coupled with adolescent pressure. Despite this, Mahiro has shown an inherent talent for composing haikus about her everyday life.

After being transformed into a girl by Mihari, Mahiro didn't put up much of a fight, though she did show some initial resistance to going outside, showing regularly, and putting up with all of Mihari's new rules for her. As time went on, her old personality started to fade away and she started exhibiting the personality of an adorable younger sister who loved dressing up and being taken care of by Mihari. The main aspect of her old personality that remained however was her love for video games, and she is seen playing them constantly in her spare time. Overall, the roles of Maharo and Mihari have since reversed with Mihari taking care of Mahiro rather than the other way around.

After being hit on the head by a box, temporarily losing her memory, and gaining the personality of a little girl in the process, some of this other personality has begun to leak through on occasion. This is shown by actions such as the occasional cutesy expression, and taking naps after eating cookies much to Mihari's frustration.

Despite multiple occasions where the drug has started to wear off (causing Mahiro's penis to regrow), Mahiro has opted to continue taking additional doses of the drug to remain female. She continues to assert her lost masculinity and rejoices when her "comrade" returns, but has opted to stay female for the time being due to loving all her new friends and her new life. She has ultimately opted for trying to find a balance between her old life and new one.

Mihari Oyama
When they were younger, Mahiro got along famously with Mihari. They would play together, and tease each other as brothers and sisters often do, but they also deeply cared about one another and Mahiro would do his best to take care of her as her older brother. This started to change after it became apparent that Mihari was a prodigy who's achievements began to surpass anything that Mahiro could ever hope to achieve. As a result, he stopped making time for Mihari and became isolating himself in his room, sleeping in past noon and playing video games while never leaving the house for a period of two years before the start of the series.

Despite Mahiro no longer caring what Mihari did, she did still care about her older brother and began an experiment to completely rehabilitate his life and make him a functioning member of society through her "Onii-chan Rehabilitation Project" which involves transforming him into a middle-school aged girl and slowly making her a functional member of society by giving her a new life as Mihari's younger sister. She thus takes pleasure in observing how the experiment is progressing, seeing Mahiro as a lab-rat for it.

Since Mihari transformed him into a middle-school aged girl, their relationship has reversed with Mihari now taking care of Mahiro. Mihari always wanted to have a younger sister, and is now taking advantage of the experiment to live out her long sought after fantasies. This is shown by her dressing Mahiro up in various girly outfits, and baking cookies for her. She also gets extremely excited whenever Mahiro acts cute and feminine, though she does get frustrated when Mahiro starts acting more feminine than she does (and tends to point it out). When Mahiro does something nice for her, or when Mahiro puts in a lot of hard work into something and fails, she bursts into tears showing how much she truly loves her and what she does.

Mahiro in turn adores her older sister now, and the two have become closer and closer, though she is occasionally inconsiderate, especially when distracted by the video games she plays.

Kaede Hozuki
When they first met, Mahiro was worried that Kaede would know who she really was, but the two started talking and found that they enjoyed each other's company. As a result, Kaede has become another big sister figure in Mahiro's new life. Kaede is always willing to help out Mahiro if she asks for it, and the two have even started keeping secrets behind Mihari's back. Kaede has even helped Mahiro and her friends pick out new swimsuits.

Etymology

 * The name Mahiro (真尋) means "true" (真) (ma) and "fathom" (尋) (hiro).
 * The name Oyama (緒山) means "string" ((緒) (o) and "mountain" (山) (yama).

Manga

 * Mahiro's Confusing New Body
 * Mahiro and the Bathroom
 * Mahiro Faces the Mirror
 * Mahiro's Self Image
 * Mahiro Plays an MMO
 * Mahiro's Melancholy Day
 * Mahiro, Even on Heavy Days
 * Mahiro and the Secret Garden
 * Mihari's Wasted Efforts
 * Mahiro's Unexpected Encounter
 * Mahiro Starts Over
 * Mahiro and the Gatcha Game
 * Mahiro and the Girls' Day Out
 * Mahiro and Kaede's Secret
 * Mahiro the Own-Home Security Guard
 * Mahiro Helps Out
 * Mihari and Her Onii-Chan

Anime

 * Mahiro's Confusing New Body
 * Mahiro's Time of the Month