Who are the main characters in "The Fly" by Katherine Mansfield?

The main characters in "The Fly" by Katherine Mansfield are "the boss" and old Mr. Woodifield.

This particular story of Mansfield's is considered by many literary critics as her darkest story because it is a tale of internal crisis as well as a criticism of sending young men off to war. In this story, the fly which represents death plays a symbolic role.

The plot revolves around the visit to a friend by old Woodifield, whose name suggests the Battle of the Argonne Forest, a major battle of the First World War which lasted from September 1918, until the Armistice of 11 November 1918. In this battle, many American and European soldiers were killed because of their lack of skill and military experience. Among those who have died in this war are the sons of both Woodifield and his friend, known as "the boss." The boss, who intended for his son to take over his business, keeps a photograph of his son taken six years ago on his desk. 

After the boss gives the shaky Woodifield a drink of whiskey, he is able to remember what it is that he wishes to tell the boss:

"I thought you'd like to know. The girls [his daughters] were in Belgium last week having a look at poor Reggie's grave, and they happened to come across your boy's. They're quite near each other, it seems."

This news jars the boss into the reality of his boy's death. No longer does the son seem to his father as the young man who lies "unchanged, unblemished in his uniform, asleep for ever." With this image shattered, the boss sits benumbed at his desk after Woodifield departs. He decides to look at his son's photograph, but "[I]t was cold, even stern-looking," and not as he wants to remember his boy.

Dead to any emotion, the boss toys with the life of a fly who has fallen by chance into his ink pot. After putting the fly on his blotting paper, he is impressed with the fly's ability to clean the ink from itself. So, the boss cruelly replaces it in the ink pot more times so that he can watch it revive, but it finally becomes too weakened, and it dies. When this happens, "such a grinding feeling of wretchedness seized him that he felt positively frightened."