Cú Chulainn, originally named Sétanta, stems from Irish mythology as a demigod hero of the Ulster Cycle. Born to the mortal Deichtine and the sun god Lugh, he earned his legendary epithet "Hound of Culann" in youth by killing a smith's ferocious guard hound and offering to serve as its replacement until a new one could be trained. Trained by the warrior-woman Scáthach in the Land of Shadows, he mastered combat arts and acquired the demonic spear Gáe Bolg. There, he formed a deep bond with his foster brother Ferdiad and fathered a son, Connla, with Scáthach's rival Aífe. Connla later died by Cú Chulainn's hand due to a tragic misunderstanding, fulfilling a geis forbidding his son from identifying himself. His most famous exploit occurred during the Cattle Raid of Cooley, where he single-handedly defended Ulster against Queen Medb's invading forces while its warriors lay disabled by a curse. He employed guerrilla tactics, single combat challenges, and his supernatural battle frenzy, the ríastrad or warp spasm, transforming him into an uncontrollable monster. His life ended when enemies exploited conflicting geasa, forcing him to either break his taboo against eating dog meat or refuse hospitality. After consuming the dog, weakened, he was mortally wounded in battle and tied himself to a standing stone to die upright. In the Fifth Holy Grail War, Cú Chulainn manifests as a Lancer-class Servant. Initially contracted to Bazett Fraga McRemitz, he is forcibly transferred to Kirei Kotomine after Kirei's betrayal. Bound by stolen Command Spells, he performs reconnaissance and eliminates witnesses under Kirei's command, despite disdain for his methods. His actions diverge across the war's routes: aiding Saber and Shirou against Gilgamesh in Fate; rebelling to protect Rin Tohsaka in Unlimited Blade Works, using his final moments to kill Kirei; and being killed by True Assassin while investigating Ryudou Temple in Heaven's Feel. He appears in spin-offs like Fate/hollow ataraxia, assuming a relaxed role, fishing in Fuyuki and attempting to charm women while working part-time jobs, avoiding direct supernatural involvement. In Fate/Grand Order, he is summonable by Ritsuka Fujimaru for Grand Order conflicts. He also appears as a Stray Servant in Fate/Samurai Remnant, allying with Miyamoto Iori during the Waxing Moon Ritual. Cú Chulainn qualifies for multiple Servant classes: Lancer, Caster (due to runic magecraft learned from Scáthach), and Berserker (reflecting his ríastrad-fueled rampages). His Noble Phantasm, Gáe Bolg, possesses two techniques: a thrown version reversing causality to strike the heart, and a melee thrust unleashing barbs within the target. His runes offer versatile combat applications. Geographically, his power amplifies in Ireland, potentially granting additional abilities. His personality blends a playful, flirtatious demeanor with a fierce warrior ethos. He enjoys leisure like fishing and banter but relishes challenging fights and respects worthy opponents. Loyal to just causes, he chafes under dishonorable masters and often seeks personal freedom. In comedic media like Carnival Phantasm, these traits are exaggerated into recurring gags, particularly frequent, often absurd deaths followed by indignant protests.