Hmm. I think my main prerequisite (apart from dark hair and dark eyes) is being under-appreciated by canon and the people around them. Thus Mohinder, Xander, Gunn, Faith, and why I went off Logan when Veronica Mars became the "Isn't Logan Awesome show". Which doesn't explain Ten, but I think I need a special category for "characters played by David Tennant" :D

Unfortunately, another prerequisite is "Not covered in scary icky scaly things"...