Ymir is a villain in the Marvel Universe and an enemy towards Thor.
Ymir is the oldest living and most powerful of the Frost Giants. He was the one who formed the creation of the Niffleheim. He is an ancient being of limited intelligence, he tends to focus all of his energies towards destruction and little else.
Except for Surtur, Ymir is the oldest known being in the "Nine Worlds" of the Asgardian realm. The Frost Giants origin is lost in antiquity; however, they are known to predate the gods of Asgard themselves.
Though there are few in number because of the heavy losses sustained over the millennia in battle with the gods, their huge size, durability, and raw power make them the gods most formidable foes. They dwell in the frozen land of Niffleheim, which is north of the country of Asgard proper.
Age has not definitely diminished Ymir's power in the least. Although possessing some intelligence and capable of speech, Ymir is not particularly cunning or subtly motivated. His vast physical power has enabled him to survive and maintain his rule over his kinsmen over the millennia.
Ymir might have no weaknesses except for lack of intellect. This might be the reason why Odin and his two brothers, Ville and Ve, were able to defeat him for the first time.