Mandark is the main antagonist of Dexter's Laboratory. He was born in the peaceful land of Flowertopia where his hippie parents Oceanbird and Windbear were in a lake giving birth to him. With Oceanbird's last breath of maternal exhalation, she had finally brought her new born lovechild into the world. They both wanted their son to be good and just like them, as soon as he was born, he was struck by a random bolt of lightning which made him truly evil on the inside.
Later, Oceanbird and Windbear were up on top of a hill where they thought about what they would name him. Oceanbird and Windbear named their child Susan because Oceanbird had requested a name without the preconcieved gender connitations. The moment the two of them named him Susan, he started crying. Oceanbird and Windbear started teaching him all about their hippie beliefs and made him do it with them. Their full intent was for him to be a good and pure flowerchild and for him to believe in peace just like them. Susan however, was still truly evil on the inside and didn't like this. Just then, Susan came across a metal gizmo on the ground. He loved everything about it and was mesmerized by how gret it was. He turned it on and it was a light bulb. Susan wanted to know everything about this and to be someone who is related to science. However, his plans for said thing would have to wait, because Oceanbird and Windbear had a different idea for his development.
Oceanbird and Windbear constantly kept doing hippie rituals with him. Activity after activity and he didn't like any of them and always tried to be evil. They triedmaking him hug trees, but he ate them. They tried painting beautiful murials, but he painted demonic horrors, and they tried balancing amethysts. Years after, on Susan's 8th birthday whcih they ironically adresses to as his annual name day, Susan finally snapped because he could'nt take anymore of this. He said he was done with this. He wanted and meat, and he wanted science. Oceanbird and Windbear came to the realization that they were neglecion their sons needs and then had the idea to drop out. They tried traveling the world to try and cure his craving for evil and technology and Susan went along with this and did not deny it. Even though he wanted to become good, he still had the need to be evil.
Oceanbird and Windbear moved out of their town and into the neighborhood where Mandark lives now. Susan tried to make friends with Dexter but he laughed at him for being girly. Susan was so angry about being made fun of that he decided to become an evil genius named Mandark. Mandark became evil and wanted only to destroy Dexter and his laboratory too. Mandark even went out and did so after he told the story but when Mandark was about to get to the door, Oceanbird and Windbear stopped him so he could put down the weaponry and do hippie rituals with them. Mandark said that even though he was here with Oceanbird and Windbear now, he would get his revenge on Dexter someday.
He is in many ways the Anti-Dexter: while Dexter is an incredibly knowledgeable knowledge with an otherwise ordinary life. Mandark is an openly fiendish inventor who is not motivated so much out of curiosity as he is greed, hate and outright malevolence. He share some certain similarities with his younger sister and they are both the opposites and rivals of Dexter and his sister Dee Dee, the latter he is madly in love with.
Mandark takes great joy in inflicting misery on Dexter but due to his obsessive love for Dee Dee, Mandark has at times allied with Dexter - though these unions are rare and doomed to failure due to the two having very different views on the world.
Mandark is arguably seen at his most evil during the one-hour special episode / mini-movie known as "Ego Trip" in which he progressively grows worse in varient futures: to the point of becoming an outright monster by the end of the movie who engages in a "final showdown" with his nemesis, aided by an entire army of variant Mandarks from many different time-lines.