Does anyone else have any mysterious figures in their lives?

I have a close friend who I have known for around 15 years and yet I know very little about her personal life. We share a friendship group through uni, and nobody in our group has ever met, heard from, OR SEEN, any relative, or romantic partner in her life, though she has mentioned them to us, so it’s not like she doesn’t have any. From the few details we have heard, she doesn't come from a good family. She didn't even attend our grad ceremony, even though she got a first. That was really odd.

I have met a handful of her friends outside of me/our friendship group, but no more than like 2-3 times. And very briefly each time. She keeps us very separate from them. Nobody has ever been to her home. Nobody has ever even seen what her house looks like, except for on Google Streetview, because I wanted to know. Nice house, not shabby or something anyone should be ashamed of. She has next to no online presence, so you can’t even try to snoop on her online. She also avoids photos like the plague despite being a nice looking girl. Always has.

A few years ago I kind of distanced myself from her because I felt our friendship was uneven. I share my entire life with her: she’s invited to every party or event I am; I mix her in with my other friendship circles; and she has met every bf I’ve ever had; she’s been to my family home and met my entire family. I felt I was sharing a lot, and she was sharing nothing, and that it was unfair. I did tell her all of this and how it made me feel, kind of hoping it would prompt a breakthrough, but she said she respected my decisions and was sorry. I was angry and upset she’d rather lose our friendship instead of opening up, but I accepted it. I stopped speaking to her for several months, but then I felt her absence in my life. I eventually reached out and told her it was ok and that it was all in her time. This was 5 years and nothing has changed. It’s weird though, because despite all of this, I don’t consider her a sly, or sneaky person. The opposite. She’s extremely trustworthy and most likely the first person I’d call if I was in trouble and needed help. There's few people I'm comfortable with leaving in my house when I'm not there, but she's one of them. I have so much love and respect for her as a friend, but also frustrated so much of her still remains an enigma after close to two decades of friendship.