Speak to Rider: You aren't a domestic servant, you're a hero who fights for and protects Sakura. You lived and died in ancient times, but she summoned you using magic to fight in a war, and you survived that war and decided to remain with her because you loved her and wanted to protect her. As for the "master" thing, well, that is a relic of the system for the war you fought in. The summoners are called "masters", and the summoned are called "servants". It does somewhat imply servitude to the summoner, but not in the traditional sense, and Sakura isn't the sort of person who enforces servitude.
Plus, there is another reason for her being called your "master". I'm not absolutely sure where in your life you came from, but I think the two of you may have ended up in a sexual relationship with each other, and that relationship involves a lot of domination, bondage and hard S&M play. Hence the "master"stuff, although you take it in turns to dom.
