18+ 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.

Kyouya Nanase is an ordinary teenage boy, who moved in with the girls Kyouka and Yuuma Igarashi after the death of his parents. Yuuma 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)