Another look at Dash X
I believe that the writers felt that someone totally diferent from Marshall and Simon needed to be in the main cast.
Marshall was a regular kid who wanted to do the right thing and defend his family from the forces or weirdness.
Dash, on the other hand, had no one and no idea where he came from, and always thought more of himself. He even suggests in his debut episode that he's the "evil twin".
We don't know that much about him so we can only guess. An alien? A Frankenstein-like kid? That's what made him so interesting. Plus Jason Marsden did a great job.
Go ahead! Make my millenium!