Mark Chang is one of several characters, along with Jorgen Von Strangle, to have gone from a (minor) villain to a friend and ally of Timmy Turner. This is because Mark, after being defeated by Timmy, assumes he is one of Earth's mightiest warriors.
He was supposed to have a dark and scary voice as a space alien, but after Rob Paulsen gave him a surfer dude voice instead, the producers of the show loved the contrast so much that they stuck with it.
It is more than a year that Mark visits the land as in "King Chang", he celebrated its first anniversary on Earth.
Though Yugopotamians find beautiful things to be hideous and vice versa, he is shown to be attracted to Trixie Tang.