I am about to start knights of the fallen empire dlc for star wars the old republic. I know you can get Arcann as a companion. But I do not know how. How do I get Arcann as a companion and what choices do I need to make?
I think you are a bit confused with the different expansions.
Arcann is not a companion from Knights of the Fallen Empire, but of Knights of the Eternal Throne.
You unlock him in chapter 6 of Eternal Throne. However, to unlock him there, in chapter 1, you need to pick the light side choice when Senya asks you to trust her. Then, in chapter 6, you can unlock him.