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.

Nanase Kyouya is an ordinary teenage boy, who moved in with the girls Igarashi Kyouka and Yuma after the death of his parents. Yuma is a "Sai Taker," but Kyouka is an "Original Taker." One day, he goes with them to see a new model under testing of a magical device, based after Kyouka's. After a deadly terrorist attack, Kyouka desperately contracts Kyouya to the new device model to save his life. The device has some side effects, however, one of which is that it transforms Kyouya into a girl! Now Kyouya is secretly an "Original Taker", and he must go to the elite "Sai Taker" school, as a girl, with Yuma. Will he be able to keep enough "Life" to stay alive? Will he be able to navigate his life as a girl? What other side effects of this device lie in wait for him?

(Source: MangaHelpers)