Don't Tell Me Good Bye [Vampire Knight]
The Tsukumo family was a powerful and kind pureblood family. The two parents had a daughter, who inherited a great amount of power, that was sealed inside the daughter's body.
Tsukumo Shiki was alone. Her parents murdered for her power. She was adopted by Kaien Cross, and met Yuki. Shiki was always distant, and knew Yuki's secret. She felt as if she must protect her 'new' sister, as they both went to Cross Academy.
Shiki is treated like a normal vampire because of her sealed powers. She doesn't obey Kaname's commands, angering the aristocrats. She is always protecting Yuki, until Rido Kuran comes along, and breaks Shiki's seal.