Ah, I see. Sorry about that. :)
I have no idea about any legend regarding the name sadly, I hope someone else can help with that.
But when looking into the surname Kopper which is apparently a Low
German surname, it said that it denoted someone who worked with copper.
Therefore it is possible that someone brought in copper and this name was given to the person who worked with it. I'm sorry i can't be of more help.
Good luck. :)