Hydrangea Girl

Hydrangeoides/Claire Riggs is an ancient ghost/demon that laid dormant on earth trapped at the bottom of a lake and pierced with the Bleeding Blade until it became dislodged and it began searching for a new host.

Hydrangeoides currently lives in the body of Claire Riggs.

Life as Helenka
Helenka was a Russian witch in the far east, banished from her home in the west for rumors of dark magic. The woman traded her soul for immortality and power, paying the cost of needing to suck the life force from others.

Her magic got much stronger though, allowing her to use the souls of her victims to power herself or to use as shadows she can control. Eventually, she was defeated by the Shukketsu hāto family of ghost hunters. Her body was impaled with their magic sword at the bottom of a shallow lake, trapping her spirit and magic.

Return as Claire
She laid dormant, trapped with the sword until it became dislodged during an oil drilling operation. Nothing more than a ghostly poltergeist now, Helenka sought out her descendants, finding Claire Riggs first and possessing her body. Claire slowly became paranoid and insane as her body and mind tried to resist Helenka, eventually succumbing to her power.

Claire accidentally killed her parents, taking their life force and was found by Company. She was locked away in their testing facilities, kept in especially isolated parts of the compound to not allow her to suck any life.

Abilities
Hydrangeoides used blood magic and necronomacy during her life, selling her soul in return for power. As such she needs to drain the life force from people to support her power. When she is low on life force her body begins to deteriorate and rot.

Hydrangeoides has demon magic abilities, specifically the ability to conjure shadow like figures and command them. Her magic is dependant on her life force though so she needs to be juiced up. She also gets enhanced strength, speed, etc the more life force she has.

She can also induce death in people and has a relationship with life force as a whole.