The author seeks to study the links between friendship and process of identification at the time of childhood and adolescence. As she retraces the treatment of one of her female teenager patient in psychotherapy, the author tells the story of a really singular friendship between two girls, two soul sisters that only death could tear apart. The analysis of the transfero-counter-transferential movements leads on one hand to release the therapist from the process of idealization in which her patient tends to enclose her. On the other hand and above all, it leads to the transforming of the therapeutic relation, which is at risk of getting lost in a confused unity.