An unreliable person. A procrastinator. A careless or lazy person. Dishonest and doesn't keep to their word. They'll tell you they're going to do one thing, and never do it. They'll tell you that they'll meet you somewhere, and show up an hour late or don't show up at all. Also spelled "flakey", or "flake" in the noun form.
She told me she would send me her pictures, but it's been 3 months and she hasn't sent me shit. She's flaky as hell.