Weekes says the following: "Imagine going through this world knowing that you're different, but not knowing how. Imagine trying to find out the truth about what you are—a mage? A demon?—and never being sure that you can trust the answer, or that your friends will still support you when the truth, whatever it is, comes out.", as well as this he describes him as valuable for those who want a member "who can move around the battlefield undetected, sinking daggers into enemies who never see him coming, Cole is your guy."
As well as this, it seems as though your companions are rather opinionated in regards to his presence with Sera refusing to talk to "it" too, which shows the complexities that companion interactions may behold, as well as being an optional companion.