To embellish based on the fact that you said the relationship isn't completely transactional:

I can tell that for my mistress -- I'm devoted to her 24/7 and our relationship is no longer transactional at all -- there's definitely not a single word. When she introduces me, she simply does so as her best friend (which makes me melt). In reality, I'd say that I am, probably not in the right order, her slave, friend, mentor, and worshipper. There's just no single word that does a decent job at covering all of those, imo:

slave: self explanatory, I'm her 24/7 slave. We live at a distance so most of the time it's long-distance slavery, but once a month, it's 3-4 days straight in person
friend: in every sense of the word, I'm the person she shares her triumps with, looks to for support, and loves to spend time with
mentor: I'm a couple of decades older than her, and the person she looks to to help guide her and coach her through life, for feedback on important decisions, etc
worshipper: maybe not the right word, but aside from the slavery aspect, I do truly worship her -- in a figurative sense -- and it's such a powerful and important part of the relationship, it's worth mentioning

Anyway, I can't see how to get that all across in one word