You have to die enough times and get all dialogues with Reaper so he gets included and you get the poly route with him and Trapper :,D
I realized that later after looking back at the walkthroughs and noticed how much we have to die so we can actually pursue Reaper 😅
You'll notice when you have to stop dying in an area if the next dialogues with Reaper/death scene do not change anymore (probably. idk, I'm guessing for now since the Waterfall walkthrough isn't done :,) )