Reported herein are the findings from a grounded study probing the impact of graduate student mentors in a National Science Foundation (NSF)-funded research experiences for undergraduates (REU) program (2014-2016) in chemistry & chemical biology at an urban private research university on undergraduates' development through the lens of self-efficacy and science identity theory. Mentor traits that appear to be most important include availability, promoting students' independence, being a good teacher, partnership, benchmarking, and demonstrating trust and respect for the protégé. These traits appear to influence recognition, competence, and performance in science identity theory. Identification of critical mentor traits and their significance to the protégé's career development should be useful in the design of effective graduate student mentor training programs.