Klaus's (Robert Sheehan) power is most easily explained by the "hello" and "goodbye" tattooed on his hands: he's a human Ouija Board. Klaus has struggled with his ability to commune with the dead all his life, using drugs and alcohol to tune the dead out. Towards the end of season one, Klaus discovers that he can actually manifest the dead into tangible beings, allowing Ben (Justin H. Min) to fight alongside his siblings. We see more of this at the beginning of season two, with Klaus using the ghosts of soldiers to fight the Soviets along with Ben. Klaus is also able to let the dead possess him, which Ben takes advantage of, leading to one of the most ridiculous fights I think I've ever seen.