Gabriel is the protagonist of the story. He is a teacher and a writer accused of being a "West Briton" (pro-England). The story of "The Dead" is largely concerned with Gabriel's tensions and turmoil as he spends the evening at a party thrown by his three aunts.
Gabriel is oppressed in a number of ways, assaulted verbally as well, and feels generally unhappy at the party. Later he is shocked into an epiphany (suddenly learning to empathize and break out of his self-concept) when his wife confesses her love for a dead man, Michael Furey.
Michael Furey is Gretta's lost love. He dies "for Gretta" when she goes off to a convent.
You can find more character information on the eNotes pages:
Gabriel is the main character, and Michael is the guy who died for Gabriel's wife Gretta...