Little is known of the past of Ted Knight's archenemy. However this much is known.
His first name is Kyle, though his last name is unknown. He was Canadian, and served in the Army during WWII, where he won a medal for bravery which is one of his most cherished possessions. After the war, he turned to work as a scientist, but his discoveries were dismissed by the Allied Forces. It is this rejection that is believed to have caused the Mist to turn to crime.
Using a machine of his own design that utilized one of his discoveries, Kyle was able to alter his body so that he could transform into a light fog that could induce drowsiness in those who breathed around him. It was in Opal where the Mist was first caught by Starman. The two became archenemies and began a life-long feud.
It was this feud that caused the Mist to have "one last crimewave", the goal of which was to rob Ted Knight, now a superhero with an open identity, of all that he valued. To that end, The Mist's son and namesake, Kyle killed Ted Knight's oldest son David and attempted to kill his younger son, Jack.
Kyle failed in this later task and was in fact killed by Jack Knight. The news of his son's death drove The Mist mad, and he was committed. His daughter Nash used his same machine to give herself the same powers and became committed to living up to her father's legacy and avenging her brother.