"My only princess" seems like a bizarre line to me, though. Does Fai know a lot of princesses? What's the significance of Sakura being the only one?
I don't think he known any princesses, honestly, perhaps only a prince at one point in time. So, he has never "served" or "cared" for a princess.
This puts more backing behind Chii as well. Meaning, I think, that Chii and Fai DON'T have a romatic relationship. If they did, it would have been possible for him to see her as a "princess". However, since Sakura is the only one he has ever treated her as such, this puts more evidece out that Chii resembles more of a pet to Fai, then as a lover.
^.~
Thats what I got out of it anyway.