Personas are a widely used tool to keep real users in mind, while avoiding stereotypical thinking in the design process. Yet, creating personas can be challenging. Starting from Cooper's approach for constructing personas, this paper details how behavioral theory can contribute substantially to the development of personas. We describe a case study in which Self-Determination Theory (SDT) is used to develop five distinctive personas for the design of a digital coach for sustainable weight loss. We show how behavioral theories such as SDT can help to understand what genuinely drives and motivates users to sustainably change their behavior. In our study, we used SDT to prepare and analyze interviews with envisioned users of the coach and to create complex, yet engaging and highly realistic personas that make users' basic psychological needs explicit. The paper ends with a critical reflection on the use of behavioral theories to create personas, discussing both challenges and strengths.