Outlanders

Outlanders (アウトランダーズ, Autorandāzu) is a manga series written by Johji Manabe.

Plot
Outlanders tells the story of alien Princess Kahm and Tokyo News Photographer Wakatsuki Tetsuya as they try to save the Earth from the invasion forces led by Kahm's father, emperor of the interstellar Santovasku Empire. The Santovasku use giant organic spaceships, or "Biomech," their advanced technology and their mastery of sorcery in their assault.

Tetsuya learns from Kahm that her ancestors in fact came from earth and they've returned to reclaim their sacred home planet that mankind has subsequently spoiled (due to pollution). However, the invasion of the "sacred planet" is really the front for a struggle between two secret fractions from the Santovasku Empire, ancient rivals which have vowed to destroy each other no matter what the cost - even if this includes all of humankind. And Earth, too, has her share of sinister conspirators who seek advancement for their own benefit only ...