I think Furkas is his name anyway's he is some sort of Companion member. He wont leave me alone in Whiterun. He draws his sword and initiates dialogue yet his options are only for asking about the companions. If you run away say to the other side of town he runs up to you and does the same thing. Attacking him results in a duel with no guards turning hostile and no crime gold added. I can't kill him without Essential tags gone but I don't want to install that mod just to deal with this [censored].

you could try

player.setrelationshiprank 4

after clicking on him in the console, this should make you friends. Save first though, just in case.

Farkas (and Vilkas) are part of a general guard faction and will exhibit this behavior if you have outstanding bounty/fine in any hold. Check around, pay your fine and he'll stop following you with his sword drawn.