Especially in chronic diseases, such as shoulder impingement syndrome (SIS), good self-management is important for patients to take personal responsibility for their treatment and make informed decisions in rehabilitation processes. Mobile apps integrating game design elements have great potential to increase patients self-management skills. A total of 21 functionalities for a self-management app were derived from semi-structured interviews with six patients and three therapists. Thereby, welcome messages, a personalized home-screen, and training plans are rated as particular useful. Ten of these functionalities could be implemented in a first prototype of SISco: your shoulder impingement syndrome companion. SISco provides possibilities for creating and executing training programs, learning about SIS, checking daily challenges, and making diary entries. Thereby, SISco motivates patients via progress bars, collecting XP, unlocking content and Dr. SISco the virtual therapist. Future work includes the implementation of further functionalities and the evaluation of SISco for usability and user acceptance.