This paper describes the design and development of an embodied conversational agent (ECA) that provides a social interface for older adults. Following a user centred design approach, we implemented a multimodal agent consisting of a virtual character and a robot. This so-called "bi-bodied conversational agent for elderly" was iteratively refined and developed through participatory design and rapid prototyping in 3 consecutive focus groups with a total of 21 elderly users. In addition to the two bodies, a Wizard-of-Oz control panel was developed, enabling researchers to control both bodies so as to respond to the user's instructions, questions, and remarks. The research resulted in a platform that can be used for future research on elderly-robot and elderly-avatar interaction. In addition, the research resulted in insights about elderly users' preferences regarding the appearance and design of a virtual and a robotic ECA (Embodied Conversational Agent), described in results that can be reused in future experiments involving ECA for elderly users.