In my initial counseling, my NCO says he needs me to counsel, train and mentor a PV2. Does this make me the PV2's first-line supervisor?

This is an interesting question. There may be a good reasoning for this but more information is needed. So, if your Sergeant instructed you to do this I would do it correctly. Your NCO should show you how to counsel a Soldier first. He/she should prepare you with the proper regulations and policies governing this process. As for mentoring a Soldier as a E4, that's questionable but if so you should have your Smart Book or Leaders Book to use as a guide. I'd contact the SGMs on LP and go offline with them for specifics.

Kinda sorta but not really. The NCO will always be the official FLL, but you could very well be that buffer to assist the FLL, to get you ready for those stripes and be an official FLL

It seems as if he is using you as an informal leader- perhaps developing your for stripes.

Who do you think trains an ammo bearer to eventually become a machine gunner?
You train the people beneath you and learn the job of your superiors. It's called "leadership".

YES! If Your NCO ‘tasks’ you with ‘this Duty’, then you are ‘in charge’ of the PV2 SGT JARED Dennis! Check the ARMY REGS! I was AG Corps! Connect for further assistance!
Nancy

If YourcNCO ‘delegates’ this ‘Duty’ to You; then You are ‘in charge of’ the PV2!
Does this answer Your question SGT

need more info on this, but you are in a promotable status, thus, you are going to counsel and mentor Soldiers anyway - I would see this as professional developement and a learning experience. In my experience, I have found that Junior leaders are very lacking in their counseling skills, and seem to be "checking the box" if they are even doing monthly counseling.

Yes