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Gustav's goal with all of this was mostly to gain new power, he wanted to infuse with life all these places so he could later on be able to fuse with their World Origin Cores and gain more power, the same way he did with Gaia and now with Mars.
There was a thing that interested him though, apparently he didn't had to just limit himself to Earth's animals, he had not thought this through enough but he could infuse enough mana in the planets to force mana-based life to emerge, mostly creatures such as monsters, and he even began to introduce monsters that could survive these environments better, even the Sun now had life, as there were many types of monsters that lived in volcanic areas in there, freely swimming in the pools of plasma like nothing…
Check "The Protagnist's Sister Is Actually the Strongest"!