Scáthach is a warrior-queen from Celtic-Ulster mythology. She is the queen and gatekeeper of the foreign territory, haunted realm called Land of Shadows. She is a prodigy of spearmanship and Rune magic. She possesses a power immense enough to rule and close the gate of the Land of Shadows, which is overflowing with countless ghosts. It has been said that she became the mentor of Cu Chulainn—who would later grow into the hero of Ulster—guiding him, teaching all sorts of techniques, and even bestowing his favorite magic spear. She also instructed his son, Connla.

(Source: Fate/Grand Order Wiki, edited)