Arsat is the hero of the story, although a case could be made for his brother. His brother helps Arsat take the woman he loves, and he sacrifices himself to protect the couple. Arsat is filled with remorse over his brother’s fate, whom he idolizes. But the brother is not really a character in the story; Arsat’s memory of him may or may not be accurate. What is real, however, is Arsat’s love for his unnamed lover, his sorrow at her sickness and death, and his regret over Diamelen’s fate. His deep empathy is heroic, as is his determination to return to face his chief. His story, and his life in the isolated lagoon, takes on a kind of mythic character in Conrad’s hands. Arsat says at one point, “A writing may be lost; a lie may be written; but what the eye has seen is truth and remains in the mind!” This can be understood as a comment on Conrad’s effectiveness as a storyteller, but there is a sense in this statement of the immutability of experience that is, in my opinion, heroic.
As with much by Conrad, the answer is complex. I think that a good case could be made for Arsat's brother as being the hero for the short story. Arsat's characterization is complex enough that he can be seen as both hero and antihero. He certainly does not see himself as a hero by the end of the story, rather being filled with an immense longing and sense of guilt for both the choices made and those not made. At the same time, Diamelen could not really be seen as a heroine, for she is one that finds herself victim to circumstance and the actions of others. Arsat's brother is the most heroic, primarily because he is the individual about whom there is the least amount of question. He is loyal to his brother. He counsels Arsat to have patience in terms of his love with Diamelen, seeking to pivot him away from conflict. Yet, when it is evident that conflict is inevitable, he does his duty as a brother, helping out Arsat in his time of need. He decides to sacrifice himself so that Diamelen and Arsat can escape, and does not flinch at the brutal death he must have experienced. His characterization is the only one where there is nobility and a sense of maintaining the social order, even surrendering his life for it. I would consider him to be the real hero of Conrad's short story.