Kanashi Yurei

Before she was born both of her parents were thought to be Quirkless. Though, the truth of their abilities was revealed when they had their first and only child Kanashi.
When she was five years old she was kidnapped by a villain. This villain happened to be a notorious serial killer, that left a brand on his victims to act as a time stamp in the events leading up to the inevitable killing.
She was missing for over a year as the killer would send videos and photos of the lowering clock on her life. Eventually the time came that Yurei was put down, and left in a drop location that was accompanied by a picture of the next victim.
In the morgue her parents were called forth to identify her. Her mother confirming the identity of her lost daughter began crying over the body. As her tears fell over Kanashi Yurei's body a blinding light filled her dead veins, and Kanashi sat up from her lifeless state, revived.
From this time an extensive medical event would take place to check her parents for Quirks, and to test Yurei. In the end both of her parents were deemed to actually have hidden Quirks. Her mother had the ability to give life to the once dead, but only once. Her father had a Quirk that could use energy around him to physically boost his strength, but to activate it he had to take life from something living.
After her initial death is when Yurei manifested the ability to take life, and to give it. She could take the life source from one thing and transfer it to another in the form of healing or a power boost.
This dangerous Quirk along with her background lead the authorities and hospitals to quarantine Kanashi for her young years. They monitored her ability to control her power, and her state of being living-dead.
Finally when she was 12 she was released into a program to protect her from her former killer. She unfortunately had to say goodbye to her parents, and live a life of solitude being moved to different locations with police guard assigned to her.
She was raised in secrecy, and she lived the same way. She learned to live with lies and deceit. People would eventually avoid her because of her cold disposition even being so young. This is around when she met her friend Hikaru Sachi. His less bright mind, and good disposition attracted Yurei. It would be easy to keep secrets from him. Though the friendship was initially just to avoid being alone, she and Sachi grew close despite the secrets she kept locked away.
When time came for the duo to apply for highschools, they both picked U.A. This secretly relieved Yurei, but even so, she had a life long promise she needed to fulfill. Nobody would stand in her way.