Taehyun Choi lives with his sister Hakyung Choi. You could say they have a normal life. On the one hand Hakyung has a good job and is successful, however Taehyun isn't good at studying.

His life takes a drastic change when he accidentally learns that he and Hakyung aren't siblings. Which is why he decides to confront her, but then she suffers a serious accident and loses her memory.

This is where everything will change, because those feelings of kinship will become much more important.

(Source: MU)

Extraterrestrial beings have invaded our garden world, hellbent on wiping humanity off the face of the planet. The only hope is a specialized force of genetically-engineered female “Pandoras” and their male partners known as “Limiters,” who represent the last line of defense against a horrific alien force.

Kazuya Aoi is a Limiter who enrolls in a prestigious academy that trains genetically-engineered soldiers. Having made a promise to his Pandora sister who died in battle, Kazuya must overcome his sadness and become the best he can be. His first major obstacle is a frigid second-year student known as Satellizer el Bridget, who has remained undefeated in combat simulation. Does Kazuya have what it takes to beat her, while readying himself for an impending war?

(Source: Seven Seas Entertainment)

After experiencing tragedy firsthand, Amagi Souji does not feel that he is deserving of love or of loving anyone. It is not until a strange girl appears before him wanting to learn just what love is that he finds himself caught up in a battle between female warriors known as "Ark Processors." Will he learn to feel love again and will he be able to teach her what true love is?! Only time can tell!

(Source: MU)

From ADV:
Woo Sang High is under attack! The entire student body is being terrorized by a duo of frightening bullies who have recently been released from a juvenile detention center. Guk Do and his muscle-headed sidekick, Chung Ki, are on a rampage and they have set their sights on Saang Tae, a brawny teenager with the courage of ten men. He alone will battle the fearsome duo, but there’s only room for a few at the top, and only one man can be The Boss.

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)

The Freezing Pair Love Stories manga tells the story of how the various Pandoras and Limiters met.
(Source: ANN)