The main character of "The Judgment" is Georg Bendemann. Kafka's protagonist is portrayed as a young man who has taken over his father's business, is about to be married, and has maintained a lengthy correspondence with an old friend who has moved to Russia (a correspondence in which he carefully excises any pertinent information concerning his current life, concerned with how his friend might take the news).
Nevertheless, the story is driven not by his relationship with his friend in Russia (with whom he corresponds), nor his interactions with his fiancée. Rather, this story is driven by the relationship between Georg and his father, a relationship that seems cast in the shadow of his mother's death. Their relationship is a strained one, and although Georg's father's health is ailing, this does not prevent him from imposing his will upon his son.
In this sense, the encounter Kafka describes between father and son is one shaped by confrontation. The older man rebukes his son, leveling accusations against him, targeting both his impending marriage and his correspondence with his friend in Russia, and as the story ends, this vicious confrontation ultimately leads to Georg's suicide.
Thus, while you might say that the main character is Georg Bendemann, through whose experience the narrative follows, its central theme is the problematic and tumultuous relationship that ties him to his father and leads to his destruction.