The first part of her name, "Hina", means "young bird", while "ko" is a common feminine name ending, usually meaning "child", from the word kodomo. "Ninomiya" is broken down into "two princesses," which is a reference to her adult and child forms.
Hinako's adult form as a tall, shapely woman is what she would likely have looked like had she not been sickly as a child, although her child form is more reflective of her true emotional and psychological age. In both forms she has long dark hair (medium brown in the anime) She wears clothing that allows her to alternate between both forms, although when she transforms into an adult it becomes more form-fitting and accentuates her figure.
Hinako's personality is as variable as her form, shifting between two very different states. In her child form, she acts mostly like a small child would. She talks in a high-pitched voice, craves ice cream and similar junk foods, watches anime on TV (Doraemon is her favorite) and generally acts very spastic, childish, and immature. Despite this, she still has a strong commitment to being a teacher and feels very responsible for her students.
In her adult form Hinako acts like an elegant, confident, and somewhat vain adult woman, and is more effective in her disciplining methods because of her forceful personality, gorgeous figure and her tendency to drain and blast people who disobey her. While she is mainly well intentioned and righteous, and can be gentle, she also seems to have a more vindictive side as an adult, actually enjoying the punishment that she deals out. Her adult form is also quite sensuous and sexual in appearance. Because of this, Hinako is not afraid to use it to her advantage to take control of the situation when dealing with male delinquents. However, she retains some of her childish traits. In either form she enjoys sweets, attractions, toys and video games, eats like a slob, and doesn't properly take care of her apartment. Hinako is quite forgetful, even overlooking to bring her wallet into a restaurant, or getting side-tracked from her original reason to visit the Tendo household once she became enamoured with Soun. She can sometimes lack good judgement, once deciding to help Akane learn to swim without knowing the basics herself. She is also quite weak for what she perceives as very manly and upright grown men who are kind to her. The longer a period she stays in an adult state, the more her energetic and irrational childish personality makes itself known.
Hinako is obsessed with enforcing discipline and justice at Furinkan, and she will do so at any moment if she feels someone is being a delinquent. Her primary target in this endeavor has been Ranma, whom she wishes to take care of so that everyone else will fall in line. Even so, her dedication to teaching also means she wants him to succeed in her classes, and has made an effort to help him with his studies. They did also team up against Principal Kuno after his abusive methods of discipline made him seem like a delinquent to her.
A teacher hired by Principal Kuno for the purpose of disciplining Furinkan's many delinquent students, particularly Ranma Saotome, she has a reputation for being quite the successful reformer. This status causes a bit of a surprise when she shows up as a young girl. Her reputation, however, is justified when she uses her ability to absorb a person's battle aura, rendering them incapable of fighting back. After absorbing it, she instantly grows into a voluptuous young woman and is then capable of firing it back at opponents as a large ball of energy. She reverts to her child form upon doing so.
The reason for her strange abilities and situation is that, as a small child, she was very sickly. Happosai was raiding the nurses' panties at the hospital where she stayed at, and he "helped" her by manipulating her pressure points, changing her metabolism so she could absorb the auras of other people to increase her health. However, his only motive for doing this was so she could absorb the battle auras of the enraged nurses whose panties he stole, allowing him to make a clean getaway. This altered metabolism made her default form age very slowly, making her true age that of her adult form.
The battle aura absorbing techniques she employs can be countered by simultaneously hitting five pressure points on her body: two on her back, three on her chest. If this is done every day for one month, her powers will be canceled out permanently. The problem with the counter is that, due to the placement of her pressure points, the only way to hit them simultaneously is to grab the teacher's back with one's left hand while roughly groping her left breast with the right. This, of course, causes the attacker to look like a shameless pervert, as Ranma discovers as he tries to do so. He later abandons his efforts after he learns that he must repeat the procedure every day for a whole month.
Later on, Hinako falls in love with Soun Tendo, due to the kindly way he treated her while she was in her young girl form. However, Akane, Nabiki and Ranma were firmly dedicated against her turning into a substitute mother, even Kasumi eventually opted against her, and Soun was knocked out before he could hear her proposal. Though Soun respects her dedication as a teacher, treats her courteously, and even admits that he finds her attractive, he can't seem to shake the memory of his late wife. While Hinako claimed that she gave up in the wake of this, she still secretly pursues Soun in an effort to marry him, and has been seen with a Tanabata bamboo charm, where she wished for fun, manga, toys, and that she and Soun would be together.
Although Hinako is a recurring character in the manga, she has a minor role in the anime, relegated to a single OAV that depicts her as just another opponent for Ranma.
Hinako is able to both absorb chi from her opponents, through any object with a circular hole as a focus, as well as shoot it back at them as a charged attack. She effectively matures into an adult whenever she absorbs chi, but reverts back to her child form when she fires it off.
Although Hinako has displayed no particular regular combat skills, her unique ability renders potential opponents incapable of fighting back at all, if she successfully catches them. While she requires a focusing source in order to successfully utilize her skill, Hinako is one of the most dangerous fighters around when one is available to her. Her two weaknesses being her considerable, but comparatively lacking speed, and susceptibility to being disarmed, but she can think on her feet and improvise if necessary.
Happosai taught Hinako her unusual techniques while she was a sickly young girl in the hospital, claiming they were a specialized form of gymnastics, but also used her to help in his panty raids. He is personally wary of these techniques, and her draining even managed to defeat the ancient lech through advantage of surprise, but the latter didn't use his superior speed to his advantage, and generally avoids defending himself against women. Despite his massive perversion, his arms are also too short to simultaneously hit all of the five pressure points required to neutralize Hinako's ability for a day. This then has to be repeated every day for a month to cancel it out permanently.
Happōgoensatsu (八宝五円殺 Happo Five-Yen Satsu) Hinako uses a five-yen coin to absorb chi that is already manifest, usually an opponent's battle aura. With the five-yen coin, she cannot absorb chi from someone who is not emitting any battle aura.
Happōgojūensatsu (八宝五十円殺 Happo Fifty-Yen Satsu) Using a larger fifty-yen coin, Hinako can absorb chi even if her opponent does not manifest a battle aura. She is also capable of using larger ring-shaped objects, such as a basketball hoop, to drain chi from one or multiple opponents. Plugging or otherwise filling the hole of the focus object can stop all of her techniques. Given Hinako's lack of actual martial arts training, swifter opponents, such as Shampoo, Ukyo, and Kodachi, have also proven able to effortlessly disarm her.
Another limitation is that her ability to sense battle aura is restricted when she is surrounded by competitive spirit, such as at sporting events.
Happōtsurisengaeshi (八宝釣銭返し Happo No-Yen Coin Return) Hinako releases all of her stored chi in a large blast. While it is quite deadly, this attack can usually only be performed once since it effectively drains all of the chi that Hinako has absorbed, returning her to her normal child form. However, she is able to stay in her adult form if she has enough energy. For instance, the vengeful chi of Furinkan students once supercharged her, so she was capable of firing off multiple blasts. A shown drawback in this instance was that her personality could be severely affected/compelled by sufficiently large quantities of "sentiment-imprinted" aura. She also had a fish bowl containing two Siamese fighting fish that constantly gave off enormous amounts of battle aura so that she could remain in adult form as she fought.
- In her first appearance, Hinako's child form looks like she is about 10 years old, while in OVA 12, Hinako's child form looks more like a pre-schooler.
- In the anime, when Hinako transforms, her clothes change turn into a darker shade of yellow.