Orange

Everyone has regrets in life. So who wouldn’t take the chance to change the past if given the opportunity? When sixteen-year-old Takamiya Naho receives a mysterious letter, claiming to be from her twenty-seven-year-old self, her life is suddenly thrown into flux. The letter tells her that a new transfer student by the name of Naruse Kakeru will be joining her class, and to keep her eye on him. But why? Naho must decide what to make of the letter and its cryptic warning, and what it means not only for her future, but for Kakeru’s as well.

(Source: Seven Seas)

Included series/one-shot:
Volumes 1-5: Haru-iro Astronaut (春色アストロノート , Spring-Colored Astronaut); #1: Bessatsu Margaret 2011-06, #2: Betsuma sister 2012-01, #3~5: kakioroshi
Mami and Tomoki have always shared everything in life, they are identical twins, yet so different in character and personality: Mami tender and helpless, courted by boys to whom she doesn't never "no" for excessive kindness, Tomoki with a strong sense of duty and a great spirit of sacrifice, would do anything to see his sister happy. But there comes a time, however, when one realizes there are things that cannot be shared, especially when it is something extremely rare and unique... just like an alien?!
Volume 6: Mirai (未来)
Volume 6: Suwa Hiroto (須和弘人)
A side story focusing on Naho and Suwa's lives in the world where Kakeru passed away at age 17.

More Information: Anilist , MyAnimeList