Spectre,
Your situation is reminiscent of one I was in. A female neighbor in my apartment complex was studying for her PHD at UCLA. She met a guy through online dating, things didn't work out, but they ended up becoming very good friends. They were inseparable BFF's and she always talked about him, not because she liked him but because they spent so much time together. His name is sure to come up if you have an extended conversation due to association/amount of time she spends in his company.
After a few years, she finished her schoolwork and recently moved to the east coast for a job. She knew it was a temporary situation, and I think that's a big reason why she didn't spend much time dating others. As for his deal, I suspect he had no game and wanted to be with her, but settled for being the emotional sponge and nice guy who helped her out all the time. She didn't have a car, and he drove her everywhere, always helped her with things, brought her soup when she was sick kind of deal.
I didn't actually spend much time talking to him, so I may be misinterpreting the situation. Maybe he's just a genuinely super nice guy who wants to help people out all the time and was completely happy being platonic friends. I'm 90% sure she wanted and had a platonic relationship with him, although she did spend the night at his place quite a bit. I talked to her as she was leaving and asked "Are you guys really just 'friends?'" and she said, "Yes, but nobody believes me." I believe her. I get the vibe that she wanted nothing more, and he appeared to be OK with not having sex with her, even though they had earlier in their relationship.
Like your situation, I got a weird jealousy vibe from him. Like I'd invite her out to group events with other friends, and she would say, "I can't. He wants me to stay in to work on this paper for school." So he'd often be the party pooper, and he was anti-social himself, so if she hung out with him, I wouldn't see them whenever I organized a night out with friends. All in all, it was a strange relationship they had, but I got a strong vibe from her that she viewed him as just a friend. As she was leaving, I said something like "I wouldn't do all the stuff he does with you if we weren't having sex" and she gave me this surprised look. I think guys and girls are just wired differently.