Manga by author Akane Kotetsu

A spirit was floating in another world's dungeon, missing its memories. Eventually "he" learned to possess other creatures and using the bodies of the beasts in the dungeon, made his way to the outside world. Dragging a young female knight, a pretty young lady, and even a beautiful mercenary into the mix, our protagonist begins his great adventure in this world.

The story revolves around Taiga who doesn't have memories from more than two years ago after his parents died in an accident. He lives with his sister, Tsubame, who he finds out is from outer space with the job of protecting him as he posses the Origin Heart.