Mediumship is the ability to see the ghosts of dead people. It is an extension of Immortality.
- Nathan is the only character to have displayed this ability.
When Nathan's powers are reversed in 02x02 he gets the abilty to see the dead and speak to them. However, he did not know that the people are dead unless they tell them or if he actually sees them die.
In 02x04 he can see Ollie after he's killed. At first Nathan thinks Ollie is alright, but after Curtis tells Nathan he can't see him, Nathan figures out it's his ghost. Also, in 02x06 Kelly's ghost tells him she's been murdered.
It seems this power may have been only activated after he died the first time, as many people did die before but he never saw their ghosts. However, after he died the first time and came back to life it meant he could see other spirits who are dead as he had technically died himself.