Partnership with teachers for professional development has been considered beneficial because of the potential of collaborative work in the teacher's own classroom to be relevant to practice. From this perspective, both teachers and researchers can draw on their own expertise and work as authentic partners. In this study, we address the need for such collaboration and focus on how a teacher and a researcher performed their roles when collaboratively implementing mathematical modeling tasks within a context of in situ professional development. Using multi-tier design-based research, as a framework, a researcher worked in a teacher's classroom to implement a series of research-based mathematical modeling activities. A broad corpus of data from this interaction was analyzed, including audio recordings of interviews with the teacher, video recordings of three mathematical modeling lessons, researcher field notes and journal reflections, instructional materials, and students' written work using the principles for designing activities for teachers. The emerging roles and relationships between the teacher and the researcher were documented, as (1) the researcher implemented the professional development, (2) the teacher shared her concerns, (3) the researcher responded to the teacher's challenges, and (3) the teacher reflected on student development. As a case study of collaboration, the participants' roles and strategies to overcome challenges and achieve shared objectives can benefit teachers and researchers who plan to collaboratively implement modeling in the classroom. The study supports the value and viability of this form of in situ professional development, indicating that significant changes in teachers' thinking about their students' mathematical model development can occur in relatively short periods of time.