This longitudinal single-case study examined the diagnostic process, treatment course, and functional outcomes of an adult male patient with dissociative identity disorder (DID). It focused on three underexamined topics: the role of a close carer in assessment and treatment, the dissociative nature of comorbid substance abuse, and changes in functionality across trauma-focused therapy.

Qualitative data were drawn from clinical interviews, treatment records, and five years of contemporaneous close carer records including written entries, correspondence, text messages, voice and video recordings. The analysis integrated phenomenological, narrative, and ethnographic methods to generate a thick description of the case and to identify thematic patterns across the assessment and treatment trajectory.

Observation by the close carer provided access to assessment and treatment information that was inaccessible within clinical settings alone. Substance abuse manifested as an alter-specific behaviour serving anaesthetic functions for trauma memories, rendering conventional addiction frameworks inappropriate and ineffective. Evaluation of functionality revealed a dissociative “simulacrum” of pre-treatment functioning supported by protective alters, contrasting with a more vulnerable but more authentic post-treatment functioning following integration.

The case provides evidence for a traumagenic aetiology of DID, illustrating how severe, chronic childhood abuse shaped the system’s dissociative organisation and the trajectory of therapeutic change. The case highlights key clinical considerations for detecting DID in complex presentations, understanding alter-driven substance abuse, and evaluating treatment outcomes beyond behavioural metrics.