The first servant you summoned to Chaldea was Artoria. She’s been with you through all of the singularities. To the rest of Chaldea you two just seem to hang out a lot but once you two are alone the truth is revealed. Artoria is actually the Master and you are her Servant. You two where in your room in Chaldea. “Now that we are alone, you’re going to do what I say right? Now kneel down my servant.”