Dexter was born as a true genius. He grew up to a normal family and at very young age created his very own laboratory in which he kept hidden from his parents behind a secret bookcase. The only person who knows that he has a lab is Dee Dee. His parents at one point found out that Dexter has a lab, but that was during the attack of a giant monster, Dexter had to reveal it to them so that they can join him to stop the monster. After defeating the monster, Dexter erased their memories so that they will not remember. Dexter attends school and is at the top of his class with his studies and tests which makes some of the other students jealous.
The Sith Cauldron
Dexter first appeared in the Sith Cauldron, along with the Powerpuff Girls. When he saw Phineas Flynn building something bigger than he did, he fear that Phineas wants to ruin his inventions. So he became a new enemy of Phineas despite for Phineas wants to be his friend.
He then read to the Powerpuff Girls and Isabella an old story, which is actually a prologue for the episode, leads to the following events later: "Legend has it, in the mystic land of Moraband, there was once a sorcerer so cruel and so evil, that even the gods feared him. Since no prison could hold his soul, he was thrown by the Jedi in to a crucible of molten iron. There his demonic spirite was captured in the form of a great Black Cauldron."
He then trying to make Phineas sad, to make him can't destroy his inventions.
At the end of the episode, Dexter resolves to cast himself into the cauldron, but Phineas stops that and advances himself instead.
Dexter was later saved by Phineas from Darth Vader, who trying to attack them. This made Dexter became a new friend of Phineas.
"I will never let you destroy my inventions, Phineas. You will never destroy my inventions."~ Dexter to Phineas Flynn, as he thought Phineas want to destroy his inventions.
"Phineas, I... I just want to thanks to you for saving me from that demon. Now I knew you never want to destroy my inventions, Phineas. I... I knew my fault now. Can we... be friends again?"~ Dexter after was saved by Phineas from Darth Vader.
"Such a day for inventions."~ Dexter.
"Phineas, I know what we will do... for today."~ Dexter to Phineas.
"Are you that bad? I think you're better than that."~ Dexter to Nobita in The Sith Cauldron.