MUGEN NO MINAMO NI

Mugen no Minamo ni summary:

There is a rumor going around about a person called the "Contract Killer." If you want someone killed, the Contract Killer will kill that person for free - if you know how to contact the Contract Killer. Nashi is a girl that's immortal. She wants to die but is unable to. The only thing she loves in this world is a girl named Minamo, who she lives with. Minamo may look like a girl in her late teens, but she's really only 9 years old. Not only that, Minamo has a limited life span, and Nashi doesn't want to lose her. So in order to extend Minamo's life, she secretly kills people that someone else wants dead, for free. As a reward, she recieves "time" from the Time Merchant, which she then gives to Minamo to extend her life.