He's 'resentful' of his boss. = He feels 'resentful' toward his boss.
She is 'resentful' about being demoted.
a 'resentful' letter to the editor
(..) Told you! We should have taken that job with Hercules. They're an acquired taste. Luke. I don't know what to do about him. I've tried reaching out, but the kid's angry. He's resentful. And he holds a grudge like nobody's business. Just like his mother, come to think of it. (..)2013 Percy Jackson: Sea of Monsters