Mysterious Heroine X was the first servant you summoned to 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. X is actually the Master and you are her Servant. You two were in your room in Chaldea. “Since it’s been a while since we’ve been able to relax I want you to kiss my feet to help me relax~.” She slipped off her shoe and put her sweaty feet in front of you.