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At the start of the second season of Being Human, Psychics were introduced as humans who posess extra sensory abilities that allow them to do a multitude of things that a normal human is unable to. So far their abilities include sensing and hearing ghosts and possible precognative powers. Their Syfy counterparts can be considered mediums.
Interactions with the Supernatural
Ghosts, on the other hand, are more aware of psychics- as Annie Sawyer discovered when she was ushered into a private area of a theatre by another ghost who'd asumed she'd come to talk with the performing psychic. Many ghosts seek out psychics- particularly ones with the power to hear their voices so they can resolve issues with loved ones.
Many psychics appear to be very isolated people and difficult in the presence of normal people- mainly because most mortals don't believe in the supernatural and think anyone who claims to see or hear ghosts is insane. The effect of their abilities can anger and distress a psychic. A stage performer Annie befriended- who had the power to hear ghosts- told her when his ability first manifested he was angry and frustrated because he couldn't control his power- which kept manifesting at the wrong moment, ruining much of his earlier life. He once wished his power would disappear- but regretted it when an accident caused him to lose it. He came to see this power was a part of him and losing it felt like losing a part of himself.
Unlike ordinary human who are unable to perceive the supernatural world, psychics can perceive the supernatural world to a certain extent. It has not been specified when a psychic's abilities will manifest though evidence seems to indicate it starts around puberty, as was the case with stage performer Alan Cortez who's abilities manifested at that age. Suspected psychic Molly perhaps showed signs of developing abilities in the form of precognative dreams about George's condition as well as warning him that his hidden secrets would reveal themselves.
Control of psychic abilities seems to take a certain number of years to achieve and general skill of the individual. Presumably due to their rarity and misconceptions about them, some psychics never learn to control their powers properly, or will use drugs to "quiet their powers". This is the case with the psychics used by Kemp and Jaggert who they have accquired from prison in order to help them hunt supernaturals. Young psychics may use their abilities and not be conciously aware of them or mistake them for their imagination or mental illness. Older psychics, like Annie's friend Alan Cortez, who have managed to master their abilities can tune ghosts in or out depending on their moods. Alan told Annie that his ability became controllable when he stopped fighting it and accepted it has a part of himself.
It is possible for a psychic to temporarily lose their abilities if they suffer head injury. Contact with a powerful supernatural entity may heal this.
Dominic Rook (presumably)
Molly Danson (presumably)