Teachers as mentors have a great impact on shaping the identity of future teachers and how they will behave in the classroom. However, mentoring roles in many countries receive little, if any, training and guidance. The dialogical self theory offers a comprehensive theoretical framework for studying student teachers' mentor's identity. The main objective of this research is to study the identities of untrained Spanish teachers in their mentoring roles when advising student teachers. A combination of qualitative and quantitative analysis reveals three clusters in the sample of 35 mentors analysed: mentors oriented towards the planning of classes and development of teaching skills; mentors oriented towards the socialization of the student in practice; and mentors oriented towards pre-service teachers' professional growth. Interrater agreement was excellent, reaching a Cohen's kappa value of 0.80. The implications of the findings are discussed along with possible considerations for future mentors' training and development.