Socio-cognitive abilities to recognise and to represent individual-specificity—even in some nonhuman species—are central to human life. Using a novel philosophy-of-science paradigm, we explored these abilities over 3 years in 6 waves by investigating individual-specific behaviours of 104 crab-eating macaques (Macaca fascicularis) and the representations that 99 human observers—experts and novices—developed of them. By applying the non-lexical Behavioural Repertoire × Environmental Situations Approach, we generated 18 macaque-specific personality constructs. They were operationalised with behavioural measures to study the macaques and with two rating formats to study the observers’ representations. Analyses of reliability, cross-method coherence, taxonomic structures, associations with demographic factors, and 12–24-month stabilities highlighted essential differences between individual-specific behaviours and pertinent representations, explored developmental pathways of representations, and illuminated attribution biases and limitations of questionnaire methods.