In this paper, the author presents some ideas about what he calls “shadowy personalities (SPs)” found typically among patients with dissociative identity disorder, and discusses how they manifest themselves, as well as how they can be dealt with in clinical settings. SPs are defined as “personality states that hold anger or destructiveness”, which have following natures; anger/aggressiveness, difficulty being identified, and temporary appearance in critical situations. The author then demonstrates some prototypical SPs such as an SP that asserts itself on behalf of the host personality, an SP with a mother's voice, a competitive SP and a depressive and self-destructive SP. A hypothesis of the way SPs are formed is presented, primarily based on Ferenczi's theory, taking advantages of notions such as introjection, incorporation, and identification with the aggressor. Then a case material is presented and implications of some psychotherapeutic approach are discussed. The author hopes that this paper is informative to those who lack knowledge and understanding about these specific aspects of dissociative disorders.