Arima Miyako

The daughter of the Tohno branch family that Shiki lived with until moving to the main Tohno household. Miyako was cast as Shiki's younger sister, but it seems that the two didn't communicate much despite living together, perhaps because of the role she was given. Her mother is Arima Keiko, and father Arima Fumiomi. To digress, you can see the rough version of Miyako's victory pose in the Tsukihime Dokuhon (a.k.a. The Blue Book). By the way, the first proper art with her was released on a plastic fan distributed as a freebie at the Summer Comiket that Kagetsu Tohya was released at. That art can also be found in the Tsuki Bako set on the PLUS+DISC in the wallpaper section.

MB Official Strategy Guide: Melty Blood Glossary
§

The Charging China Girl.
A young girl that uses pretend baji quan, and (secretly) idolizes Shiki as her big brother. Currently a sixth-grader in elementary school.
Shiki came to live in her house at around the same time she started to become aware of her surroundings, and it seems that for her it was something close to love at first sight. Ever since then, she has been spinning around in circles trying to figure out how to get closer to him.
Because she's the kind of person that gets incredibly flustered when she's with the person she likes, she was always at a loss for words around Shiki, and expressed her frustrations by tackling him. Before he knew it, head-butts to his solar plexus had become just another part of Shiki's daily life. Though, after Shiki returned to the Tohno mansion it seems she was able to start talking a bit more.
In Melty Blood, she appears as a hidden character. Burning with a passion to bring back her big brother, she confronted the entire Tohno household on her own. It was one of Melty Blood's comedy routes, the showdown to determine the strongest little sister…!!
Though normally she doesn't have any actual battle ability, due to the Night of Wallachia's influence her pretend baji quan ended up turning into the real thing. It was the debut of Miyako's Amazing Baji Quan.
Her combination of a Chinese jacket with jean shorts made for a cute and wholesome outfit, but there were also rumors swirling around about “no pants” and “missing tights.” Maybe there's even a rare win pose where she takes off her jacket?

Tsukihime Dokuhon Plus Period: Tsukihime Dictionary Revised
§

The Charging China Girl.
A young girl that (secretly) idolizes Shiki as her big brother. Currently a sixth-grader in elementary school.
After his traffic accident, Tohno Shiki was adopted into the Arima household, and it seems that for Miyako it was love at first sight. However, she had no idea how to deal with suddenly having an older brother out of nowhere, and ended up repeating the same cycle of awkward communication over and over. In the years leading up to his return to the Tohno mansion, she didn't manage to have a proper conversation with him even once.
“This house has a dojo, but nobody ever uses it. What a waste!”
Thinking that, Miyako started practicing (self-taught) kempo every day. Though it's merely pretend baji quan at the level of a child's play, in Melty Blood it became the real thing for a single night due to the Tatari's influence. Of course, this delighted her to no end.
Also, she is recognized by Neco Arc as a much-awaited future rival.

MB Act Cadenza PS2 Manual: Tsukihime Dictionary