The first question has answers, but not-so-much the second one. You kinda just gotta get the feel for that yourself. Generally once you know how to create something, then you can start on making it unique afterwards. One good thing you can do though is study other character-mod creators' works around here: See what makes them tick, note what you like and don't like about different people's works (what's fun to you about them, what isn't, and the things you would change), and note the similarities between different mods (are there gameplay trends between several characters that seem to just be the same thing over and over?).
Also, your character doesn't have to have high visual quality to be considered "good". What matters most is how that character feels to play.