The Prosopography of the Byzantine World project addresses the crucial problem of the unity of the Christian eastern Mediterranean in the thirteenth century. For Byzantium, this unity depended on two factors: recognition of the authority of both state and church, after the fall of the capital city to the Latins. When the patriarchate was reconstituted in Nicaea in 1208, it set up the main religious identity factor in the Byzantine world. It was more difficult for the government of Nicaea to secure its imperial legitimacy. In this contribution I shall investigate how this regime in Nicaea succeeded or failed to establish its internal legitimacy, against the views and claims of areas outside the empire of Nicaea. My perspective will be based on a direct prosopographical approach, looking at aristocratic support for and opposition to the so-called emperors ‘of Nicaea’. The Byzantine aristocracy was always led by two complementary principles: the possession of imperial titles and local power. It is crucial to grasp the relationship between these two principles in the thirteenth century, for it allows us to study the phenomena of unity and dissent that characterized the Byzantine world at that time. This in turn permits a test case, analysing the different aristocratic groups that supported the Laskaris dynasty and the Palaiologos family that eventually gained supreme power. The replacement of the former by the latter, which occurred during Nicaea’s European expansion, suggests the possibility of multiple allegiances.