The Utrecht-Management of Identity Commitments Scale (U-MICS) is a tool for assessing three identity processes (commitment, in-depth exploration, and reconsideration of commitment) in one ideological (educational) and one relational domain. For the Japanese version of U-MICS, concurrent validity has been confirmed in the educational domain, but not in the relational domain. A possible reason was that participants could choose which person to think about when answering items about relational identity on the U-MICS questionnaire. Because associations between relational identity processes and correlates might be different across participants choosing differing persons, theoretically consistent associations were not found in total in the previous study. The present study addressed this by investigating which person is important to relational identity formation in Japanese university students (N = 642, 48.0% female, mean age = 20.0 years). We first divided participants into four groups according to the person they thought about filling items about relational identity: (a) father, (b) mother, (c) best friend or partner, and (d) sibling or other. We then examined associations between the three relational identity processes and measures of personality, psychosocial problems, and parent-adolescent relationships, by group. Results showed that meaningful associations between all these were found in a group of participants who chose their mother, suggesting that a mother can be the most important frame of reference for relational identity formation in Japanese university students. Implications and suggestions for future research are discussed.