Just give the character human qualities. Make him or her act human with believable dialogue and purpose. Think of how you will become interested to a person. Make the character complex. Add more to your character.
Just give them characteristics you see in likable people. Of course, they're also human - have them make mistakes, and regret it later. Have something sad happen to them, to give them sympathy. There's 100 ways to go about it, it's just a matter of how you want the character to be conveyed.
Give the character redeeming, interesting aspects and qualities and some character development to boot. Any of these may work: witty, funny, kind, smart, romantic, cute, pretty, sympathetic, helpful, down to earth or just being a person who makes the world go 'round or helps other people. Have them do likeable things, as that's basically the only thing people can be judged on; so make them comfort and console, help and give advice, be there for others and have some good judgement, and, voilà, you've got yourself an awesome character anyone would smile at, laugh with, adore and envy.
