The consensus on the world wide web is that the true name of Dr. Arden, as purported by Anne Frank, is Hans Gruber. Most seem to be aware of the name's previous use in fiction, so they're not completely clueless. Some go as far as to challenge AHS writers for this choice.
However, "Gruber" is not what I hear. I've listened repeatedly and I hear "Cooper" (most likely spelled "Kuepper"). I can't be certain if:
- They all misheard and substituted a familiar name with a similar sound.
- Most of them said "Huh?", did a Google search and then accepted the first claim they found from some random person on tumblr.
- I am actually completely wrong.
I've tweeted Murphy for the answer but I'm not holding out hope for a response.