Cú Chulainn Alter is the berserk, altered form of the Irish hero Cú Chulainn from the Fate series, specifically in Fate/Grand Order. Trained by Scáthach in the Land of Shadows, he wields the cursed spear Gáe Bolg. Unlike his normal self, this version is consumed by rage, embodying a mad warrior who fights without mercy on the battlefield while upholding his geis oaths. He despises betrayal and favors strong women, showing a wild, unyielding spirit as Ulster's greatest demigod hero.