Brother Leon is the pale, ingratiating, and slyly venomous Assistant Headmaster.
Having seriously abused his authority at the school to purchase (way) more candy bars for the annual sale than usual, part of what drives Leon to be so strict, demanding and ultimately cruel is the fear that he will lose his job if the sale does not go well.
It has been his decision to double the quota and the price in the annual chocolate sale, and the financial foolhardiness of this project leads him to seek a commitment of support from the Vigils.
However, it is his overbearing and domineering personality that lead him to overstep his authority in the first place.
Leon needs the help of the students to keep himself from being fired from the school, yet he maintains an abusive relationship to nearly all the students he interacts with. He knows about the boxing match at the end of the book - he is invited by Archie - and he attends the event, failing to stop the violence in the ring.
His dark qualities make Leon a villain in the book, more sinister and negatively defined that Archie. Archie at earns some amount of sympathy from the reader, but Leon earns no sympathy.