Arisaka Hatsune is a character in the series Tona-Gura!. In the complete absence of their parents (certainly in the anime, likely in the manga as well), Hatsune serves as the mother-figure to both her own younger sister Kazuki and to their childhood friends, Kagura Yuuji and his younger sister Marie. A bright and energetic individual whose beauty of spirit matches her body precisely, she does not mind serving as the mother of the two families--so long as no one ever refers to her as such. She is the chief advocate, often to Kazuki's discomfort, of the roller-coaster relationship between Kazuki and Yuuji. She is better at almost everything than Kazuki, and has a legion of admirers including a former teacher and Yuuji himself. Despite very much noticing her busty figure, even the 'perverted' Yuuji tends to respect and obey all wishes and rules Hatsune lays down. In fact, Hatsune's greatest conflict seems to be with little Marie, whose hair-trigger 'punishment' of her brother's hormonal surges is something Hatsune likes to frustrate, to the point of tying the little girl up and hiding her weapons. She is also not above using Kazuki's best friend Chihaya as a means of spying on the young couple and monitoring their sometimes glacial progress.
When the Kagura siblings return after ten years, Hatsune seems less shocked than Kazuki at the 'new, perverted' Yuuji. There are indications, direct in the anime, hinted at in the manga, that Kazuki has idealized the past and that Hatsune remembers a playful Yuuji and a less uptight Kazuki. Despite her conflict with Marie and her besting of Kazuki on nearly all fronts, she is very concerned for all the members of her extended family, actually feeling relieved when the seemingly-emotionless Marie expresses her love to her brother. Later in the manga, when Yuuji has begun to back off of his more extreme behavior, and Marie feels bereft with no place in her brother's life, Hatsune does her best to help Marie past this bad time. Any sign of illness on Kazuki's part will all but send Hatsune into a panic, even needing the Kaguras to talk her down.
Hatsune as said has legions of admirers--some forming clubs-even rival clubs--dedicated to worshipping her. But she seems most interested in possibly pursuing a relationship with her former tutor, now Kazuki's teacher, Kogorou-San, who seems likewise interested, though he too tends to become flustered around her. Hatsune's mothering instinct is not to be trifled with : Yuuji's photography boss, Tojo Haya, learned this when she forcibly prevented him from calling home when he was away during a crisis. Hatsune did not even have to raise her voice to make her point known and understood.
In the anime, it is Hatsune who shows Kazuki's childhood diary to Kazuki, revealing that her memories of that time were idealized, and that Yuuji was not perverted, so much as he had not grown past his little boy antics. She also made sure the two had a chance to talk things out when it seemed like the Kaguras were moving again, and was wise enough to pull back a little when the Kaguaras instead extended their stay--though not too much. Another anime difference is Hatsune's hair - which is brown in the anime, and blonde in most colorized manga shots. In the manga, even though Hatsune's matchmaking efforts have borne fruit, she still tends to intervene and makes herself available as a sounding board for the young couple, whose coming together she predicted in a piece of fiction she wrote early in the manga series--again to the discomfort of Kazuki. Regardless of every other factor in the series, it seems likely that the devoted Hatsune will remain her sister's number one protector and headache. She still delights in making her sister think that her affections may be more than sisterly, and in aiding Yuuji, even when he goes too far.
The final success of her efforts in the manga have yet to be scanlated.