This character is based off of a god in Japanese mythology. That makes it a great villain for Thor, seeing as though that's Norse mythology. Let's have a battle of the mythologies, people! It'll be fun! Amatsu-Mikaboshi was ruler of Earth before it even resembled Earth. Two gods came to it and decided to create life. The life they created were considered Gods to the Japanese, which pissed off Amatsu-Mikaboshi because it was the real OG god.
Amatsu-Mikaboshi tried to destroy his enemies, but they bitch slapped him down to the Japanese netherworld for a long time. His main vendetta is against the gods that sent him to hell. Amatsu-Mikaboshi pretty much takes any opportunity to mess the gods that he can. You can imagine that after Asgard fell, he went wild with no one to stop him.