I need help with an idea that I don't know how to implement.

My game is 2D, but I want to make it so that when my player opens their inventory, a reflection of the player appears in a certain area of ​​the inventory panel, like in Minecraft. I just want the player's reflection to be visible.

I don't want to move the player's layer or position; I want to create something that mirrors it without affecting the main player

I think you answered your own question.

Attach another sprite with the player's image to the inventory; this is a common practice in games.