Illustrative case material is presented to demonstrate genetic linkages between the presence of imaginary companions or fantasy twins and extensive mirror play in childhood and early adolescence, and the appearance of mirror dreams and depersonalization in later adolescence and adulthood. The defensive splitting of the self-representations observable in these phenomena is viewed as a means of warding off castration anxiety and anxiety about object loss, anxieties which arise because of conflicts primarily centering on intense aggressive drive derivative wishes. Formulations about depersonalization are discussed.