The aim of this paper is to provide a formal framework for Requirements Engineering modelling languages featuring agents, behavioural goals and operations as main concepts. To do so, we define Khi, a core modelling language, as well as its formal semantics in terms of a fragment of the multi-agent temporal logic ATL*, called ATL_KHI. Agents in the sense of concrete and provided entities, called actors, are defined by their capabilities. They also pursue behavioural goals that are realised by operations, which are themselves gathered into abstract, required, agents, that we call roles. Then a notion of assignment, between (coalitions of) actors and roles is defined. Verifying the correctness of a given assignment then reduces to the validity of an ATL_KHI formula that confronts the capabilities of (coalitions of) actors with the operations in roles played by the said actors. The approach is illustrated through a toy example featuring an online shopping marketplace.