In this article, the authors share the story of Diego, a kindergarten student, navigating issues of mathematical identity and competence--his own and those of his kindergarten classmates. The authors are a team of educators, including a kindergarten teacher (Sara) and a math teacher educator (Jody) who worked together on a quality improvement initiative associated with year-long, district-wide professional development for K-5 teachers around the topic of mathematical argumentation. The story of Diego emerged within a lesson they co-taught in Sara's classroom. The authors share what they learned in the experience of this specific lesson and what it looks like to form a mathematical identity and then zoom out and reflect on what it might mean for the classroom.