Unlike most existing work on traversable interfaces, we focus on the use of synthetic characters to accompany the user in Mixed Reality (MR) applications. We examine virtual companions as a promising means to design smooth transitions between different worlds and to avoid orientation problems. We propose a taxonomy to describe the spatial relationship between character and user which has an important impact on the style of interaction. To flexibly transfer user and character into different spaces, we have created a platform that supports the design of interfaces derived from the proposed taxonomy as well as transitions between them.