((*Stalks into thread form airvent*
Sorry, followed WJ here. BTW, Mind if I use some of your background for my char? I really want him to have some kind of connection with another of the chars and I have a partial idea for yours.))
Name: Edward Pierce
Skills: Swordplay, animal handling, negotiating, stealth.
Items: Knife hidden in boot, wrist-band made by his mother, clothes, small stone the day he first met Erin and her brothers.
Background: Knew Erin as a kid, he was an enigma in the neighborhood since he never seemed to go to a house of his own except at night. Felt partially responsible for Erin's mother's death, and swore to protect her. He then admitted to her brothers that his own parents were dead, killed by bandits, and asked them for help. They trained him in swordplay, but one night he abruptly left, not to be seen again.
After that he became a thief himself, though he only robbed from those who could afford it and never killed anyone, he was caught by one man and blackmailed. Finally he grew into debt so much, the man to whom he owned the debt agreed to let him pay by becoming his slave. He died 2 year afterward and was sold by his son to (insert whatever owns every character now)
Vader's wife was very pregnant when she died 19 years ago. All of a sudden a 19 year old who is very strong in the force and has a distinct resembalance to him in his younger days rolls in from Vader's home planet with his old Master Obi-Wan (Who was the ONLY other person preset at the time his wife died. And to boot, the kid's last name is Skywalker.
So in answer to your question, he knew Luke was his son because his name is Darth Vader, and not Darth Retard.
-Forum post on why Vader knew Luke was his son.