If you are now actively serving in uniform, it's probably true that you have a mentor or two in your background. For a young person in their first enlistment, maybe their mentor was their recruiter, or maybe one of their Drill Sergeants, or their first squad leaders. For those more senior, mentors have probably come and gone along the way, but each is still out there, having an influence in some way. A question in all this is, "Who was/is your MAIN mentor?"
Who has had the most positive impact on your professional growth and development in the military? If you can't answer that question, then you may not in fact have a real mentor—you've just had a few people along the way who pointed you in some direction or another. What I'm talking about is having someone who actually takes the time to really care about what you are doing. Someone who talks candidly to you, hitting you upside the head with things you are doing "not so good", even if you personally believe you can do no wrong.
I remember who that "main mentor" person was for me when I was on active duty, and he's still around today giving me "vibes" from time to time—well after I retired. He took his time to really care about not only me, but all those under him. Not only did his guidance help me in numerous ways, he set an example for me to do the same type mentoring with those with whom I worked.
So, to senior people serving, I say, "Be a MAIN mentor to as many as you can." To those younger, I ask, "Who is your main mentor?"