I am so tired but I'm thinking of about how the trolls are representative of the faeries in Arthurian legend. And Trollhunter changes a lot of that ofc but one thing is that there's a price and cost to everything. Jim becomes of the Trollhunter but he loses his childhood and sense of stability. Changlings can hide in both worlds but will never belong in both, and for them to thrive their human counterparts must suffer. Claire gains power but use too much and she is lost. You can't have one thing without the other. And ik I mentioned Jim's cost before but its more then that.
Throughout the series he is constantly between two worlds and at first its a bit of a joke, but by the time Merlin comes along Jim is running on empty. And his friends, they think they chose, but they are a part of Jim's world that should have stayed seperate yet bled into the magic world as Jim was dragged further in. Jim in the beginning had a sense of preservation. He doesn't want danger nor the constant violence inherent to that role. But he already is stuck in the deal and there's rarely a way out a deal with the these creatures, no matter how unwilling.
All of it concludes in the bathroom scene. Jim stands before the dark water and evaluates his life and the costs it took to get here and it isn't enough, not for the hungry tendrils of a magic. Either he gives himself to the trolls or both worlds die because he didn't pay a high enough price. And it isn't fair and it isn't kind but he steps in and he sinks and he dies and someone else comes out because staying fully human was never apart of the deal. Barbara watches her son be drawn into the wood and has to watch a different child come out.