She's Asuka the armored girl, Tobimaro's younger sister who has the unusual combination of being as timid as a mouse and having the power of a locomotive.
Asuka inherited her mother's superhuman strength, speed and agility but is a meek and quiet girl with a childlike mind. Her mother had her raised in isolation from men to keep her pure. Her brother never knew of her existence and she didn't even know what a brother was. The first man she ever set eyes on was, Ataru as he ambushed her. The experience left her with a really bad impression of men and now she has a dangerous phobia of males. From then on the mere sight of a man will send her into a frightened rampage causing massive destruction and swatting away all of those scary men.
It turns out that her parents had arranged a marriage between her and Shuutaro Mendo to unite the two families. This came as a total surprise to Mendo. The first time he laid eyes on her, all he saw of her was a figure in a hulking and fearsome black armor. Asuka likes to wear this tank-like armor because it makes her feel safe, but it doesn't make her very desirable. The thought of marrying a big tin-can didn't sit well with Mendo so he tried to squirm out of the arrangement. But when Lum accidently hit the armor with her lightning bolts, it collapsed revealing a beautiful maiden underneath.
As soon as he found out about her true appearance and saw her face to face, it happened. For the first time ever after years of leading girls on, Mendo actually fell victim to love at first sight. All of the sudden he was very motivated to marry Asuka. Mendo began fighting over her with Ataru and this really frightened Asuka. But her brother Tobimaro nobly defended her repeatedly against the two lechers. Strangely enough because he had starry eyes like her and because he had come to the rescue, Asuka fell in love with her own brother, who she had never seen before that day.
Poor, innocent Asuka has a very mixed-up view of males. Asuka doesn't understand how male-female relationships are supposed to work. She'll often try to sleep with and take baths with her brother, not knowing what a huge taboo it is. You see when she was about to be introduced to the world, her handmaidens did a pretty poor job of trying to describe men to her. Despite their best intentions, they made them sound scary, while also referring to big brothers (read Tobimaro) who are a "special kind of man" who look like her and are all very nice. So Asuka seems to assume that all men are scary with the exception of a special breed of man called "big brothers" (onii-sama) that are angelic figures of virtue.
By the same token, when Asuka hears Ryoko refer to her brother as "Onii-sama" she seems to believe that Mendo must be a big brother too and is soon sorry for treating him like he was a man. Even Ryuunosuke, because she wears the same samurai-like bandages over her torso that Tobimaro does, is seen as a big brother too. So in Asuka's eyes, she now has three big brothers, although maybe they'd be better off if they weren't liked by her. Being loved by Asuka is perhaps more dangerous than scaring her because one hug from her can crush a man's spine.
Asuka's mother tries everything to cure her girl of her phobia. But everytime Asuka takes a step towards getting over her fear, along comes Ataru who triggers a relapse.
Voice: Sumi Shimamoto
Asuka's seiyuu is none other than the incomparable Sumi Shimamoto. Many may know Sumi as the voice of Kyoko Otonashi from Maison Ikkoku. Other very notable roles include Clarisse from Cagliostro's Castle, Nausicaa from Nausicaa of the Valley of Wind and Gin Rei of Giant Robo. Sumi is a true veteran of anime but remains one of my favorite voice actresses even to this day because of her sweet and endearing voice.