Watanabe-no-Tsuna is a Saber-class Servant from Heian-kyou Singularity.

Identity

One of the Four Heavenly Kings serving under Minamoto-no-Raikou, who participated in "Mount Ooe's oni extermination", slaughtering the oni after having them ingest poisoned alcohol. He fought Ibaraki-douji during the incident, and she was the only survivor. Ibaraki later assaulted Watanabe, either at Rashoumon or the bridge of Ichijo Modoribashi depending on the account, and he severed her right hand.

(Source: TYPE-MOON Wiki)