Someone explain what is the caller and core posessor again? ಠ_ಠ) haha im so confused

basically the core possessor has powers and it gets more powerful as they get closer to the caller, (the caller is selected from the core possessors most important/significant person in their life at that time). UhhH I think the farther the caller leaves then the core possessors powers will decrease, and at some point they die? Not sure if im getting the terms mixed up

Does that mean the MC and the soldier guy (lol i forgot their names) known each other before? Didnt they meet for the first time at the forest? ( •ˍ• )
Thank you for explaining!