Scáthach

A warrior-queen from Celtic-Ulster mythology. The queen and gatekeeper of the foreign territory—a haunted realm called Land of Shadows, a prodigy of spearmanship and Rune magic. 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. Also instructed his son Connla. (Source: Fate/Grand Order Wiki, edited)