Kevin. Kevin? Yes, Kevin. Kevin Callis. Chances are he is as flawed a character as all the males in Lost are (and Kate seems to be attracted to disturbingly flawed characters), but as her girly fantasy alt "Monica" she really loves Kevin Callis. What is there not to love? He is intelligent, kind, funny, caring, trusting, and as good looking as Nathan Fillian in the very short appearance of season's 3 "I Do". And he provides the perfect mother in law as well, which is a rare find.
Main character? Well in that case my $ is on Desmond. Definately the sanest of the insane, and with promising potential. The classical literary hero.