E-coaches are promising tools for health interventions, being scalable and relatively low costs compared to human coaches. Nevertheless, they cannot compare to humans when it comes to trust and social companionship. Hence, many researches were held around the importance of tangible user interfaces in building trust and companionship during human-machine interaction. Co-design, in particular, is the act of creating with stakeholders within the design development process to ensure the results meet their needs and are usable. This chapter describes a design process method and the rationale for building an embodiment for a companion and a trustable e-coach. We also shed lights on the co-design process held in the NESTORE project, which was specifically conducted with older adults to build a vocal assistant that supports user’s behavioral change, as well as to increase the social bonding with the NESTORE e-coach.