How can Winston be considered a heroic figure? What qualities does he possess that could define him as one?

Winston Smith can only be considered a hero in a highly qualified sense. His mixture of the heroic and the mundane is indicated clearly by his name, which combines "Winston" from Winston Churchill (famed for refusing to accept defeat when fighting against seemingly invincible enemies) and "Smith," the most prosaic of surnames.

His heroic traits are his willingness to fight against impossible odds, risking his life in the process, and his stubborn quest to understand not only what was happening to him and society, but why it was happening. Although he tries to "get along" as best he can with the society he lives in, he holds himself apart from it at the same time, attempting to preserve his individuality. It would be ungenerous in the extreme not to concede his struggle some elements of the heroic.

However, he is a deeply flawed hero. To begin with, he is himself part of the apparatus of repression, being engaged in the rewriting of history. He goes too far in his desire to attack the Party, compromising himself morally when he promises O'Brien that he will harm children if he is ordered to by the resistance. He cannot separate his political struggle from his personal feelings, which lead him to become entangled with Julia, friendly with Mr. Carrington, and trusting in O'Brien, all of which decisions are fatal. His thought and actions are already infected by the Party, and since he is thus compromised, he cannot reach a fully heroic stature.