Scathach is a recurring demon in the Shin Megami Tensei series.
"A warrior queen of the Land of Shadows in Celtic lore. She taught the Irish hero Cu Chulainn the art of war, and gave him the spear Gae Bolg."
Scathach ("Shadowy") is a figure in the Ulster Cycle of Irish mythology. She is a legendary Scottish warrior woman and martial arts teacher who trains the legendary Ulster hero Cu Chulainn in the arts of combat. Texts describe her homeland as "Alpi", which commentators associate with Alba, the Gaelic name of Scotland; she is especially associated with the Isle of Skye, where her residence Dún Scáith (Fort of Shadows) stands.
Scathach's instruction of Cu Chulainn appears in Tochmarc Emire (The Wooing of Emer), a foretale to the great epic 'Táin Bó Cúailnge'. Cu Chulainn and Emer fall in love, but her father Forgall forbade the union until Cu Chulainn has completed his warrior training. Cu Chulainn and his friend Ferdiad travel to Dún Scáith, where Scathach teaches them feats of arms, and gives Cu Chulainn his deadly spear, the Gáe Bulg.
Cu Chulainn begins an affair with Scathach's daughter Uathach, but accidentally breaks her fingers. She screams, calling her lover Cochar Croibhe to the room. Despite Uathach's protests, he challenges Cu Chulainn to a duel; Cu Chulainn dispatches him easily.
To make it up to Uathach and Scathach, Cu Chulainn assumes Cochar's duties, and becomes Uathach's lover. Scathach eventually promises her daughter to him, without requiring the traditional bride price. Scathach also grants Cu Chulainn the "friendship of her thighs" (they have sex) when his training is almost complete. When her rival, the warrior woman Aífe, threatens her territory, Cu Chulainn defeats her in battle and forces her to make peace. Aífe also sleeps with Cu Chulainn, producing his son Connla, who Cu Chulainn kills years later without realizing he is his son.