A character that has already come to his own conclusions and is going to sometimes challenge my choices. A character that will ultimately make a stand, regardless of what my character decides. (Jolee Bindo's proclamation "I am a Jedi" and drawing his saber against my dark-side KOTAR character is the most powerful side-kick conversation I can remember.)
Unexpected humor and warmth is another big thing for me. (Mordin Solus)
Pure, all-in, uncomplicated characters can also be a riot. (HK-47)