Clock King: Dexter once bought an expensive popsicle and paid with pennies. When the ice cream man asked Dexter how long it took him to count the pennies, Dexter correctly answered the question.
Crouching Moron, Hidden Badass: Of a sort. Dexter tends to fail spectacularly at any form of physical activity, particularly sports... until the backdrop changes from "sports" to "Deedee let a giant monster out of the wormhole machine." Basically, he's an exaggerated stereotype of nerds are wimps up until science is involved. He can also go from deflecting lasers with his wrench like a Jedi to slap-fighting his rival and somehow manages to be in character both times. Rule of Funny is involved, but still.
Grade Skipper: Dexter once went to college. At first, all other students only wanted to party and he refused to join, despite their warnings about getting it out of the system. By the time Dexter proved them right by buckling under the strain, they were the ones who only wanted to study for the upcoming finals. His attempt to make a wild college party happen anyway got him expelled.
In another episode, Dexter was considered for grade-skipping but needed to improve his P.E. grades. He cheated his way into victory and ended up becoming Dee Dee's yearmate.
Height Angst: In "Ewww, That's Growth", Dexter and his family want to go on a really big rollercoaster, but Dexter is denied due to being too short. The resulting angst drives him to become as tall as possible, eventually allowing him to finally go on the rollercoaster by the end of the episode. This backfires when he becomes so tall that he hits the ceiling at the very end.
Ambiguous Disorder: Sometimes has hints of this- she lacks a lot of social intuition, can't take a hint, seems to be very temperamental, and even her own friends get frustrated by her odd behavior at times.
Nice Girl: She's pretty darn sweet and she absolutely adores her little brother. Her destructive tendencies are usually due to clumsiness or carelessness rather than malice, and sometimes it's shown that the reason she's always sneaking into Dexter's laboratory is because she just wants to spend time with her brother.
Evil Counterpart: To Dexter; Tall and composed to Dexter's short and insecure. His younger sister, Olga "Lalavava" Astronomov, is this to Dee Dee, a short and solemn Technician to Dee Dee's tall and joyful Performer; while Dee Dee unwittingly destroys Dexter's creations but truly loves her brother, Lalavava bullies Mandark mercilessly.
No kidding, he was just originally portrayed as Dexter's rival and at best, just wanted his lab shut down. Suddenly it went from that to trying to take over the world.
To a lesser extent, his crush on Dee-Dee.
Also, he didn't do freaking everything in the same rhythm as his Evil Laugh when he was first introduced.
Freudian Excuse: His parents were people perpetually stuck in the 60's (i.e. hippies) and constantly stifled his scientific curiosity. He tried his best to go along with it, but one chance meeting with Dexter on the day he moved into the neighborhood ended with Dexter laughing in his face. Needless to say that was the straw that broke the camel's back.
Younger Than He Looks: He's (presumably) the same age as Dexter (though it's implied that Dexter has skipped at least one grade), but he's as tall as (if not taller than) Dee-Dee, and could easily pass for a teenager.
Berserk Button: As seen in "Dexter's Rude Removal", the most Mom ever got beyond angry was when the rude clones of Dexter and Deedee not only make a huge mess of her kitchen during a food fight but using profanity as well.
Mistaken for Badass: She once "defeated" the reigning snowball champion (Dexter's dad) by playfully chucking a snowball at him. He's been convinced ever since that she has evil snowball powers, which she passed on to Dee-Dee.
One-Shot Character/One-Scene Wonder: She only appears in a very early episode of the series and never shows up again after that. But look up any fanart of her and you'd be surprised at how much of it there is.
Talking the Monster to Death: Convinces Simion, another super-simian, to abandon his plan of destroying the Earth in favor of swinging around in a jungle. Not bad for somebody who can only make monkey noises.