Personal deities are not protectors or spirit guides. The yidam is chosen, often by a guru, to correspond to a tantra practitioner's own personality. The yidam is visualized in meditation practice as an aid in inner transformation.
Scholar and translator Francesca Fremantle wrote that a yidam is not an external being. "The real meaning of yidam is entirely internal and psychological; the yidam is the expression of one's own basic nature, visualised as a divine form in order to relate with it and express its full potentiality." (Francesca Fremantle and Chogyam Trungpa, The Tibetan Book of the Dead, 1975, p. xvi)