Mash was the first servant you met at Chaldea. 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. Mash is actually the Master and you are her Servant. You two were in your room in Chaldea. “Since you’ve been staring at my feet so much Senpai, I want to start worshipping them for me.” She slipped off her shoe and put her nylon covered feet in front of you.