Think about what makes a regular person likable. Is it an infectious laugh, a kind soul, beautiful smile, knowing the right words to say at exactly the right moment, the perfect snarky comeback?
Sometimes you build a character you don't care if other people like. Other times you need someone who is only appealing to the masses during particular scenes or with particular people.
Something that helps me when I'm trying to fill in a character is going back to basics. Why is a powerful question. Katie hates snakes. Why? One slithered into her house as a child and woke her up slinking up her leg. Would Leo not use razor blades during his pain tolerance training? Why? He would have to see someone with cutting scars so he knows the trainee won't have the same aversion to the cutting and he could do serious psychological damage for a training exercise. Hadley hates impressionism. Why? Because it's too easy for the masses to appreciate. Stephon likes it when Hadley is mean to him. Why? Because it's the only time he feels really deficient tactically and because he can be just a mean back.
~Raging and quivering female mass of hormones and tosser of Dark Side Cookies™ (trade marked by Etoile)