Reman (the Worldly God) is a very important god in Cyrodiil. And probably the whole Tamriel. I was wondering why he is missing in Skyrim? Or does he have another name in
Nordic region?

He's not missing. Nords wouldn't have as much reason to revere him as Imperials, and Reman was mentioned a few times, particularly in getting into Sky Haven Temple. Also, while he was called the Worldly God, he never became a god in the sense that Tiber Septim did.

Reman is a God-Hero, not really a God-God. He's like Emperor Zero or the thousand hero/animal/nature worship cults of the Nibiean. The 'Imperial' religion is really alot more complicated lore wise than it is game-wise and you don't get a good feel for Imperial culture. Remen however is an Imperial 'God' if you will, not a nordic one, and he isn't apart of the official pantheon.