She is a cousin of Shiki Tohno, as well as his other step sister. She received a slight mention in both Tsukihime and Kagetsu Tohya, then made her first real appearance in the Melty Blood fighting game series.
The Arimas are relatives to the Tohno family, but have very weak demon blood compared to them. When Shiki was adopted into the Arima family, Miyako became close to him, despite being several years younger than Shiki.
Keeping with her “China Girl” theme, Miyako seems to wear a traditional Chinese outfit. Her clothing in Melty Blood also resemble traditional Chinese dress.
However, one of her victory poses in Melty Blood shows that she wears a T-shirt and shorts inside.
Although she tries to look strong, Miyako is actually quite shy, especially around Shiki. One of the routes in Melty Blood story mode shows that Miyako has a hard time saying anything to Shiki. Because of her frowning reticence, Shiki mistakenly assumes that Miyako is always angry at him for some reason.