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Dexter had at first a secret laboratory in his house, where Dee Dee usually broke into and caused chaos. One day Mandark broke in with his Jackbots and tried to destroy Dexter's lab. One of his Jackbots shot towards Dexter but Dee Dee defended him and was hit instead, killing her. Dexter couldn't handle it; he nearly killed Mandark, and started to build an android replica of Dee Dee using human organs in order to make the android as human as possible.
Dexter attended the Megaville Elementary school and met the Powerpuff Girls there. During recess Dexter got into a fight with them, but he eventually became friends with them. After that he mostly just hang out with Blossom when he fell in love with her. It was revealed that he was keeping a secret laboratory in the school as well, at the library. It could be opened by picking up a certain book from a shelf. When Blossom saw Dexter entering it, she followed him and saw Dee Dee's Cyborg, but he demanded her to leave.
After Blossom got captured, Otto reported that to Dexter, who went along with Coop in Megas to Mandark's Laboratory to save Blossom. When he was there he broke into Mandark's lab and started to fight with Mandark. After Dexter beated Mandark he freed Blossom, but he was too late since Blossom already died. When she was dead she was encountered by Grim who was ready to take her with him, but the spirit of Dee Dee came and asked Grim to let her stay alive and so he did. Dee Dee asked Blossom to take care of Dexter until she was back. Blossom was alive again and Mandark told Dexter that he also had a sister, and during his explaination he became mad and thought Dexter wanted to kill his sister. Mandark summoned a giant Cluster that attacked Dexter who became unconsious. Buttercup came to save him and defeated the Cluster, Mandark then activated many bombs to kill them and himself. The Megas came in the building to save the girls and Dexter and so they escaped while leaving Mandark behind. When she was at home Blossom explained to Dexter that she once also had a sister but since she was unstable she died.
A few days later Dexter was in a battle with a globmonster, but failed and was saved by the girls. At school Blossom attended dance lessons and Dexter came too. Although he got depressed after seeing Bubbles dance. Blossem noticing this suggested they leave, which he agrees to. Olga sees him when he's about to leave and asked if there was a genius who could fix the light and Dexter replied he could do that. After fixing it Blossom told Dexter that the real name of Olga was Lalavava Astronominov, which made Dexter feel shocked because he found out that Olga was the younger sister of Mandark. After that he started to acting weird towards Blossom. Some time later the whole gang, including Dexter, went to the beach. Although everything seemed normal there were some tensions among Dexter, Blossom and Olga. Olga and Dexter got into a discussion whether Mandark had died or not, Dexter blamed himself for it and apologized. He promised he would fix the broken glasses that Olga found in the wreckage of her brother's lab.
At the science fair when the Clusters attacked Dexter fought the Cluster along with Mrs. Honeydew. He also saved Olga as he thought he was now responsible for her. Dexter was helping the scientists open the door to the safe room, but needed help from Bubbles and is about to fight a Voltron made out of the Cluster bugs. When Bubbles was screaming to Olga to go get him, he came to her instead and had told Bubbles that they have a serious problem to look into. Dexter explained to Bubbles that he can't hold off the Voltron Cluster any longer and that they immediately need to close that door to the safe room. Dexter was seen inside the safe room with Olga watching Bubbles struggle to keep the Cluster monster away from the door. Then when he noticed that the bomb went off, Dexter is starting to feel worried about Blossom because he loves her. Having thought that Blossom had met the same fate as his sister Dee Dee, Dexter started to cry while Olga tries to comfort him. As the explosion spreads, he and Olga braced each other and, as the explosion reached them, they kissed each other good-bye.
When Blossom is preparing to fire her optic lasers at the central generator where Gir is planting the bomb in, Dexter is shown in Blossom's flashback, talking about how emotions can affect what kinds of outcomes people face. He also stated to Blossom that controlling one's own emotions can make one see things more clearly, making one's understanding and decision-makings more easier and quicker.
After the explosion from the bomb vanished, he, Olga, and everybody else noticed they have survived. After checking the data on the after-effects of the bomb's explosion and overhearing Bubbles saying that Buttercup and Blossom survived as well, Dexter started to feel greatly embarrassed because he kissed Olga (Accidentally as he tought Blossom was DEAD) when the bomb exploded.