Name of romance novel?

It was one of those old paperback historical romance novels but I don't remember what the cover looked like. It was about a woman who grew up with either a fairy or a elf guardian, body guard, something like that. He is a fairy/elf so he is either immortal or at least lives a very long life so he is an adult when he meets her as a child and again is either her guard/teacher/babysitter I just don't remember but I'm pretty sure he is present in her life as she ages. There isn't anything gross, they don't develope feelings until she's an adult. They aren't supposed to get into a relationship though, it's against the rules for fairies to be with humans (pretty sure)... I THINK in one part it says he is supposed to be like a teacher, then brother and finally a son to her but that obviously doesn't happen. I read it at least 10 years ago at the library I have no idea what it is called, who wrote it, nothing.