So we are like spirits... In the planet we lived on the planet itself was like our body. In this world we need individual bodies. I take a willing participant who will die without my spirit. I moved here because the alien enemies destroyed the two twin planets. I consider earth mine in a way...
Birth: Another one of us is born by combining remnants of a spirit that thinks it lived long enough combined with the water from the planet. It may be possible for me to pass on a part of my spirit to a human child.
Death: I can only be killed with the spirit of the enemy planet with higher soul ranking by it interacting with my spirit. I can only kill an enemy by interacting one of lower ranking in their planet. In similar ranking spirits it is a bit complicated the two spirits will harm one another. It would not be good for them to share a body because human bodies are so small. It would hurt both of them.