This is a spoiler unless you have already read the first two books!
Padan Fain was a
Darkfriend and an
Andoran peddler who made regular journeys to the
Two Rivers district. He was a petty
Darkfriend but because of his circumstances the
Dark One selected him for greater work and took him to
Shayol Ghul, where, after much torment and torture, he was given the ability to sniff out
Rand Al'thor and the urge to find him. He was also made into a figure of great authority among the servants of the
Dark One. When chasing Rand he was thoroughly miserable and began to resent his lot, but was still compelled to perform to his utmost. He chased Rand and his mates into
Shadar Logoth, where
Mordeth himself attempted to take over his body. He failed, but Fain is now a combination of the two, and is no longer bound to the
Dark One.
He is by far the coolest and most interesting character in The Wheel of Time series of books, because you see him become just so damn evil, and ruthless and totally insane, yet he is now just as much against the Dark as he is against the Light. He reverses the roles normally taken in the series - when he engages a Fade in eye contact, it is the Myrddraal who becomes frightened.
He has no qualms about torturing or pillaging but his victims are, as often as not, Darkfriends and Shadowspawn. He is not physically strong, does not have any of the Dark one's power and cannot channel, yet he is his own master and a force to be reckoned with. He uses trickery and deception to gain information from the Darkfriend networks, when it's normally them doing it to someone else.
Above all else, he has his own priorities, different to the rest - he does what he does, not for the fate of the world or for personal reward, but because he's driven to do so and feels he must take revenge.
When the Horn is being hunted for, he could really care less - he is after a dagger from Shadar Logoth. He is still very much a baddy rather than a goody though, and has more than enough hatred for Rand, whom he blames for the misery he experienced as a Darkfriend. From a reader's perspective, we could do with a little more of him and his thoroughly frightening madness that he brings to the books and Robert Jordan hasn't written enough of him in them.