While at first he seems well mannered and eager to help, Mike Morningstar is a vain and greedy villain in the Ben 10 franchise. Mike Morningstar drains energy / mana off of people thus himself more powerful. However, after being defeated by Ben, Gwen and Kevin he becomes ugly from losing all his newly gained power and needs a mask to hide his face. While wearing said mask he goes by Darkstar due to his darker demeanor and mostly tries to regain his beauty. He briefly had a thing for Gwen and then Charmcaster but both ended poorly to say the least.
In the rebooted series, he is more of an energy vampire that is hundreds of years old that feeds off of people's affection of him. His relationship with Charmcaster is not much better.
In the rebooted series Drake Bell plays Morningstar in his first appearance but all other appearances he is played by Yuri Lowenthal.