Nayuta Tenkawa

Nayuta Tenkawa (天川 なゆた, Tenkawa Nayuta) is a mysterious girl being cared for by Mihari Oyama's faculty advisor Chitose Azuma. A loner with an unknown background and no social life who wishes to be Chitose's research assistant, she is enrolled by Chitose at the same middle school as Mahiro Oyama during Mahiro's second year in order to monitor Mihari's experiment without her knowledge. She befriends Mahiro after revealing that she knows the truth about her, and quickly joins Mahiro's group of friends.

Appearance
Nayuta is a young second-year middle school girl. She has long shoulder-length blue-colored hair with prominent bangs and green eyes. She also wears a small gold bear-shaped hairpin on the right side of her hair. In more casual situations, she wears a beanie and places the hairpin on it instead of her hair.

She is also notably not very well developed as a female, but is further along than Momiji and Asahi, with her bust being only slightly larger than theirs.

Personality
Nayuta is a smart, but shy girl who begins to come out of her shell after making joining Mahiro's group of friends.

Having spent so much time alone reading in the university library, Nayuta possesses a lot of knowledge for a girl her age. It is unknown how well this translates to her schoolwork, but it can be assumed that this has helped her greatly. It is entirely possible that she is a prodigy much like Mihari, which would explain her interest in being Chitose's research assistant. She was subsequently seen to be overjoyed when given the opportunity to help Chitose with her research.

When Nayuta was first introduced, she had no friends and was uncomfortable interacting with people her own age. Mahiro provided a compromise as she was technically an adult, and Nayuta found it easy to talk to her, especially after revealing that she knew the truth about her. Mahiro's friends soon welcomed her into their group, and Nayuta shed a tear over having finally made friends. She has since been seen interacting with her new friends as any girl her age would.

Using the tablet that she carries around, Nayuta keeps tabs on everyone she meets. This doesn't bother her friends very much, but they were surprised when they first saw it. Despite knowing everything about them, Nayuta is very secretive about her own life. She speaks cryptically, and tends to avoid answering personal questions by claiming that the answer is confidential.

Nayuta cannot swim, and her nearly drowning is a running gag throughout the series. Even with a paddle-board or a life preserver, she somehow manages to still fall under the water. She herself thinks that it is because she hasn't developed as a woman, and that somehow having larger breasts provides better buoyancy, though this theory was ultimately proved false.

Early Life
Nayuta has a mysterious and unknown background. She is very cryptic about herself when asked, and it has even been suggested that she isn't even a child. What is known is that at some point Chitose Azuma started taking care of her.

For an unknown peiod of time afterwards, Nayuta would spend all her time in the university library reading the books while Chitose did her research. Mihari Oyama noticed her several times but never learned who she was.

All that Nayuta wanted was to be Chitose's research assistant, finding it difficult to make friends her own age. Chitose declined her request due to her young age, but Nayuta never gave up.

Etymology

 * The name Nayuta is a Buddhist term referencing a very large number, over 100 million.
 * The name Tenkawa means "heaven" (天) (ten) and "river" (川) (kawa).

Manga

 * Mahiro's Big Secret (Part 1)
 * Mahiro's Big Secret (Part 2)