The Messenger of Allah (sal Allahu alaihi wa sallam) said: “When Allah gathers the first and the last on the Day of Resurrection, a banner will be raised for every betrayer, and it will be said, ‘This is the betrayer of So and so, the son of So and so.’” [Muslim]
A betrayer is one who makes a promise then does not fulfill it, or whose loyalty and support is counted on but who does not deliver it.