Na no Nai Shisha

A boy who knows a persons' "time until death" and has the special ability to give away life; an existence known as Shisha (Messenger).
This nameless boy meets a human girl, Kumi, who gives him the name Teku, and gradually teaches him things like happiness and sadness. However, Teku, who can never become an adult, and the growing Kumi are bound by the inevitable fate of partings.
I wonder if I'll ever meet a human I wouldn't mind offering my life to...