From CMX:

Matsuyuki Takeru could not have anticipated the decidedly different turn his life would take. After his mother's death, life continued at a reasonably normal pace — he studied hard for exams, played fantasy role-playing games, even fell in love.

But things began to change when he discovered a half-sister he didn't know about. A strange boy started appearing, claiming to be a warrior reborn who must fight an epic battle against forces of evil! When fantasy becomes reality, will every choice he makes affect the outcome?

Collection of 3 short stories, the first two are Please save my earth related, but they are standalones and spoiler free: PSME-Things accidently left behind PSME-BIRTH Cibi-01 no YaYaYa! (Cibi, right, not Chibi)

In the Kioku Senmei series:

V.1 - Kioku Senmei
In the year 20XX the vast city of New York is under constant terrorist attacks at public places. The police is clueless and the culprit is roaming the city free. The police’s only witness died, and her clones are useless, so they are left with no choice but to receive the help of Jojo Schmit of the EPIA section, a section dedicated to ESP related phenomena.

V.2 - Guuzen ga Nokosumono (Things Accidentally Left Behind)

• Kioku Senmei II

• Birth

• Cibi-01 no Ya! Ya! Ya!

From ShoujoMagic:

Living in regret that his formula gave birth to the atomic bomb, Professor Einstein sends two boys whose eyes can see the future on a train that travels at the speed of light before embarking on his own final adventure: death. His hope is that they will find "HER" and tell her his wish, for it is only she who can grant it.

Fifty years later, it is 2005 and the place is Tokyo. At last, "SHE" (Ruika) has been found... but Ruika has an unusual circumstance. For her mother's sake, she's been living as her dead brother since childhood. Does Ruika truly have the power to grant wishes? If she grants her mother's wish by becoming a boy, then no one can lead us into the GLOBAL GARDEN... Now it's a race against time and against each other, because everyone has a dark past and an ulterior motive for possessing the girl who can grant Einstein's wish.

From MegKF:
This 4 volume manga is about Haruko, who, after her mother's death, gains her mom's familiar and her witch's name. Unfortunately, Silk, the cat familiar, has been unable to teach Haruko any magic but dream communication. But beware, if anyone finds out her witch name, she'll have to obey that person. Is this real, or is it all in her mind?

From Viz:

Are the woes of the present meant to be endured for the promise of a better future or to atone for past wrongs? In a captivating epic of love, fate, and faith, seven young people must struggle with this spiritual puzzle as their alien past lives threaten the future.

Since the conclusion of the original series, Rin and Arisu are leading happy lives. Their son, Ren, is a very normal boy. But one day, a man and a woman as beautiful as an angel appear in front of him and tell him about a power! What is this power Ren suddenly has? The much anticipated sequel to "Please Save My Earth"!