This paper details a framework for mixed reality agents, i.e. agents that exist in both the real and virtual space. These agents combine the physical presence of a robot with the adaptability and expressivity of a virtual character. The objective is to blur the traditional boundaries between the real and the virtual and provide a standardised methodology for intelligent agent control specifically designed for social interaction. We show how this architecture can be employed in the context of a mobile collaborative mixed reality environment that is cohabited by both robots and humans. As an example application we describe how the framework can be applied to a museum guide that takes advantage of the physical and virtual presence of the mixed reality agent to convey an individual and personalised learning experience. A mobile robot with associated virtual persona is the gateway to this mixed reality experience. The physical robot navigates the museum, while its virtual persona, which is unique and can be personalised for each observer, explains the exhibits and adapts its appearance to match the current context.