Well, Guts had not been born either than so was not Dara. But consider Dara had born, and lived, with her family longer than Guts ever did so she may have heard of a man who had bright red hair and ruby eyes (two traits which Guts had got from his father). So that could work for him and bring interest to Dara as he wants learn much as he can about his father (and his human mother too) as he never knew him.