Tsuda (津田) is speculated to be the real name of Sasaki Kojirou.
In Fate/hollow ataraxia, Tsuda (or Tsushida for the English translation) is the name Sasaki introduced himself as to Yukika Saegusa when they first met, since he wanted to conceal his true identity and the fact that he was a Servant.
In Fate/Requiem, Makki, a character said to have survived the Fuyuki Grail War, refers to her samurai servant as "Tsuda" while he refers to her as "Yukika-dono", heavily implying that she is an older version Yukika that formed a contract with Sasaki.