We discuss the design of the Intermediary Agent's brain, the control module of an embodied conversational virtual peer in a simulation game aimed at providing learning experiences regarding the dynamics of collaboration at the inter-personal (IP) level. We derive the overall aims of the game from theoretical foundations in collaboration theory and pedagogical theory and related requirements for the virtual peer; present the overall modular design of the system; and then detail the design perspectives and the interplay of the related operationalised concepts leading to the control architecture of the Intermediary Agent, that is realised as a simple cognitive appraisal process driven by direct and indirect effects of the missionoriented and social interactions of players and agent on the agent's level of trust in its human peers. We conclude with coverage of related work and insights from first deployment experiences. Categories and Subject Descriptors I.2.11 [Artificial Intel...