Many children experience challenges with school mathematics during the transition from primary to secondary education, with several studies documenting a decline of performance, motivation, and self-efficacy. In order to understand how early adolescents construct their mathematical identities during this transition, this study explores, along the lines of grounded theory, the ways that identity manifestations are expressed by children in Oslo, Norway. Participants were 21 early adolescents between the ages 12-13, during their last months of primary education (year 7), and right before their transition into lower-secondary education (year 8). Data collection involved focus-group interviews with all children, as well as individual interviews with six of them. Thematic data analyses yielded three themes around which participants construct their mathematical identities: popularity, effort, and achievement. Aiming at further unpacking the complex relations between the three themes, we focus on the case of Tina. We conclude with suggestions for further research and implications for practice.