Clearly, in this great short story by James Joyce, the protagonist is Gabriel. He, like all of Joyce's characters in his collection of short stories, Dubliners, experiences an epiphany at the end of the tale, when he realises a central truth about himself and humanity at large. However, this only comes in the last page of the story. Before that, it is important to focus on how Gabriel is presented. He is a character that seems obsessed by the way that he will appear and come across to others. He is unsure of himself, and seems ridden by a form of paralysis that prevents him from doing what he thinks he should. Note how he is concerned about quoting poetry in his speech, wondering if his audience would understand the allusion:
He would only make himself ridiculous by quoting poetry to them which they could not understand. They would think that he was airing his superior education. He would fail with them just as he had failed with the girl in the pantry. He had taken up a wrong tone. His whole speech was a mistake from first to last, an utter failure.
Obviously he is preoccupied with potential failure and how others view him. We see these same characteristics again and again, as he thinks about responding to Miss Ivors, but does not want to risk a "grandiose phrase with her." In the carriage with Gretta after the party, he thinks of making a romantic advance, but doesn't act on his desire.
However, what ushers in the epiphany that Gabriel experiences is the discovery of his wife's first love, Michael Furey. Gabriel is forced to accept that he has never loved anyone or experienced love in the same way that Gretta loved Michael before his death. This makes him reflect that it is far better to live life fully rather than live as if you are already dead, which is what he has been doing by his constant preoccupation with how others view him:
One by one they were all becoming shades. Better pass boldly into that other world, in the full glory of some passion, than fade and wither dismally with age. He thought of how she who lay beside him had locked in her heart for so many years that image of her lover's eyes when he told her that he did not wish to live.
Gabriel experiences his own inadequacies and is forced to confront them head on as he compares himself to Michael Furey and finds himself a pale comparison at best. The constant reminder of inevitable death in the story convinces him that he needs to live life "boldly" rather than to quietly "wither" into nothing.