Is there something from the past that haunts your character? Maybe a skeleton in the closet, a deep, dark secret that no other living person knows.
Has your character developed right amount of empathy, humor, and maturity to be relatable or even likeable?
What about culture? Virtues, beliefs, morals? How about sex appeal, charisma, charm. Is your character strong on the inside as well?
How does your character identify, and can we identify with them? Can we sympathize with them? Can we love them? Do we loathe their very existence? Perhaps we see our lesser attractive qualities in them and we identify that way.
Are they vulnerable, afraid, or fearless and powerful? If your character is in trouble, will we want to see them get saved?
Lastly, has your character been broken before? Are they battling their own demons? Are they fallible and do they make mistakes?
It may be a real life character, or pure fiction, but to really develop a character you must harness the essence of their humanity, the very fabric of their being. Every right, every wrong, every failure, every victory, and all the boring and mundane life in between. Warts and all.
Authentic characters and great characters.
So give us characters we will care about.