Luke had forgotten about it, but he had to give some sort of explanation regarding Nambra to his parents. They were the only ones he owed explanations to, after all. Still, it was hard to give any explanation about her and since she was so clueless and didn't show much reaction to anything, she probably would cause more problems if she opens her mouth.
"She doesn't look like Rina and Luthi," Julia added.
It was pretty obvious that Nambra was an elf from another world. The white hair was enough to make anyone suspicious. Still, the purple color of her eyes and extremely pale skin were other important factors. In any case, lying wouldn't do him any good, so Luke decided to be sincere… until a certain point. There was no need to reveal too much since it would cause misunderstandings in many aspects, it also would raise more questions and he didn't have time for that.