I had been sledding with my lifelong friend and enemy Percy Boyd Satunton, and we had quarrelled, because his fine new Christmas sled would not go as fast as my old one.
Boy Staunton's early introduction into Fifth Business immediately sets him up as a villain through the narrator's choice in descriptive vocabulary: i.e., "enemy." Later, when Staunton appears again, his actions and attitudes confirm the reliability of the narrator's ten-year-old ("at which time I was ten years and seven months old") opinion of him as an enemy.
While the boys are returning from their quarrel over sleds, Percy aims yet another snowball at Dunny, the hero of the story, and, because Dunny dodges it swiftly, the snowball hits the pregnant Mrs. Dempster in the back of the head causing her injury and a fall.
While Dunny felt overwhelmingly guilty that his action had been the unintentional catalyst for Mrs. Dempster's injury and sorrow, Percy felt no guilt nor acknowledged any responsibility over the incident at all.
His brazen-faced refusal to accept responsibility seemed to deepen my own guilt, which had now become the guilt of concealment as well as action.
This effectively sets Percy Boyd (Boy) Staunton up at his introduction into the story as the villain--and his further actions take him downhill--just like his sled--from there.