The Story of My Friend Who Gives Away Her Lifespan

Miyuki and Hikari are two gals who have reached their third year in high school, and they are childhood friends who've been stuck with each other for a long time.

They spend each day having fun doing dumb things, but Hikari has a special power: she can see another person's lifespan, and she can give some of her own lifespan to someone else.
Miyuki remembers telling Hikari that she didn't want her to use her power, but somehow Hikari's lifespan keeps getting shorter and shorter.