First, many catholic saints are proved to exist. You may not believe in miracles or any other religious elements, but they are certainly not fictional.
Second, though Mary Mother of Jesus has no historical registers but there are several reasonable causes for that: she was a woman on a highly patriarchal society and romans barely bothered to register the history of people who they conquered. We barely know about her for the same reason we barely know about many women of the same age. What we have in the Bible and in catholic oral tradition is, from a materialistic point of view, an exaggeration of people's deeds.
Now in the case of people like Joan D'Arc, we have legitimate historical register. Her behaviour was heavily documented and monitored because of the role she exercised, well more than many political figures.
So, I think catholic saints should be put in the "real life" section because the fact is that those people existed. You may say people exaggerated their personalities, but this may be the case of many historical figures. At least for saints we have more than just the Bible.