The protagonist's specific formation
The bildungsroman's protagonist must have a specific character at the beginning that is genuinely different from their character at the end: specific limitations that the novel will dismantle, specific strengths that the novel will develop, specific misunderstandings that the novel will correct. This specificity is the difference between a bildungsroman and a novel in which a young person has experiences: the bildungsroman protagonist is formed in a specific direction by specific experiences, and the reader should be able to identify both the starting point and the ending point of that formation clearly. The protagonist's starting character should be sympathetically rendered — their limitations should feel like the natural limits of their age and experience, not simply flaws — so the reader can root for their development.