Sai:Taker - Futari no Artemis

In the future, an energy source called "Life" has become incredibly important. Certain people are able to synchronize with devices called "Sai" which gives them the ability to extract "Life" and have super powers. These people are known as "Sai Takers." Within the "Sai Takers" there is an even smaller group, "Original Takers." "Original Takers" have a contract with a magical device, and they can manipulate "Life" without the use of a "Sai." These powerful people are also immortal, and they only die if they run out of "Life." The world admires "Sai Takers," but "Original Takers" are careful to keep their identities secret.