Ymir is the first great adversary Buri (Odin's grandfather) encounters. Ymir's sons kill Buri out of jealousy. Odin and his brothers then go to war with Ymir and his sons and kill them all but for Bergelmir and his wife, who leave to settle new lands, now Jotunheim.
Features to include when drawing:
Shades of blue/blueish skin color
Hair is black or white
Yellow, white, or black eyes
Facial hair optional
About the same muscle tone as the Gods
Generally scary looking
Helmets and armor in a warrior style, for his helmet use ram's horns
Weapons can be axes, war hammers, swords, or the like. Just style them differently than the Gods.
Ymir represents and personifies, and is the source of influence of jealousy and envy.
Ymir is a main character (villain) in the beginning of the Norse, of Course! story "Heimdall" as well as in Norse legend and mythology.
Don't depict him as "giant" literally, or gigantic, but roughly the same size as the Gods. Think larger than life, leader/king of a realm or kingdom. The mythological depiction is a metaphor for reputation, symbolism, and/or what he is known for.