It's a bit hard, but first you have to sit with Lake and chat with Rune and Devon, then choose the option to tell Devon about your key, after that you will have to choose the forest (Klaus), and in the part where you have to look for the planet and choose to sleep with Devon, wait for him and go with Rune and Jorgen, after that the game will give you an option to follow Klaus, choose yes and that starts his route (if I have an error about the message it's because English is not my first language sorry)