This one was difficult to me because I had never been a mentor, but I think a great answer would have been to say that I was one to my younger sister, and then give an example of when she was experiencing girl-related milestones and that she had someone to look up to for help feeling more comfortable in her own body. VS's major values are about feeling good in the skin you're in, so anything that incorporates that idea. This goes for other questions like "how did you make a customer's day better in a previous job position".