I’m having a problem with word choice in the specific sentence below.

“Batman has a friend named Robin who always helps Batman when he is in danger. Very few people know who Batman really is ______ Robin.(except/besides)”

Based on the context, I consider that Robin does know the real identity of Batman, so he should be included in the group of those people who really know Batman. If the “very few people” part were changed to “a few people”, I would be confident that “besides” best fit the blank.
However, when it comes to “few people”(which, in my point of view, should be considered as nearly no one), I’m not sure whether Robin should be included in or excluded from that very group.
So can you help me with the word choice here? Thank you very much.