Lucifer asked Rouge about the sixth son's detailed information about how he looks like.
Of course, he meant everything literally that he can use to identify the sixth son when he tries to find him.
Rouge quickly answered Lucifer by telling him what he knows about the sixth son and how he looks like.
It wasn't hard for him to disclose this information since...well, his loyalty has already gone over to Lucifer ever since that nightmare.
And it was also because Rouge was able to meet the sixth son quite a few times before.
No, in fact, he already met everyone in the Warmester Family. If he didn't meet them, he wouldn't be able to identify them for Lucifer when they were searching for them earlier, after all.
That was why it was easy to disclose this information because he knew about how this sixth son looks like.