A myriad of studies has investigated the association between personality and relationship outcomes, such as relationship stability, satisfaction, and quality. However, the importance of processes interposed between personality and relationship outcomes has been largely neglected. Our study intends to address this shortcoming by integrating both distal factors, such as core and surface characteristics, and proximal factors, such as cognitive, emotional, and behavioral processes, in a longitudinal design with couples of all ages. Proximal processes are understood as mediating variables between personality and relationship outcomes, unfolding their impact over time. Moreover, we aim to uncover how relationship processes entail both partners’ personality development, for instance by increasing stability in core characteristics or promoting change in surface characteristics.