I cannot find a particular theme/trait/aspect in most of my SUs, if only because I often try to mix up the qualities of my characters for the sake of variety. If anything, most of my characters are generally calm, sometimes witty or sarcastic, and more often than not oppose authority (with the exception of my character in Atlantis). I generally make my characters free spirits who are generally carefree and start from there. Then I try to see how it can be interpreted in different ways with the hopes that I can warp the character into something more. It is how I made my trio of characters: Isaac, Lyle and Raymond. Each character represents how free or independent thought can define an individual.
tl;dr: All my characters are male and most are relatively carefree. They are "happy" characters most of the time. Unless they are alcoholic.
EDIT: Woo! 800 posts. Only took me 4 years.