Cu Chulainn is an Irish mythical hero who appears in the stories of the Ulster Cycle, as well as in Scottish and Manx folklore. The son of the god Lugh and Deichtine (sister of the king of Ulster), he was originally named Setanta, but gained his better-known name as a child after he killed Culann's fierce guard-dog in self-defense, and offered to take its place until a replacement could be reared. At the age of seventeen he defended Ulster single handedly against the armies of queen Medb of Connacht in the epic Tain Bo Cuailnge. It was prophesied that his great deeds would give him everlasting fame, but that his life would be short – one reason he is compared to the Greek hero Achilles. He is known for his terrifying battle frenzy, in which he becomes an unrecognizable monster who knows neither friend nor foe. He fights from his chariot, driven by his loyal charioteer Laeg, and drawn by his horses, Liath Macha and Dub Sainglend.
Cu Chulainn appears as a distinct demon in Shin Megami Tensei III: Nocturne. He cannot be encountered nor recruited by the Demi-fiend, rather, the Demi-fiend can only obtain him by evolving Setanta by reaching at least Lv. 52 himself and completely mastering Setanta's skillset.
"A demigod hero of Celtic mythology who originated in Ireland. He is the son of the sun god Lugh, of the Tuatha De Danann. His name means "hound of Chulainn" A strikingly handsome young man, Cú Chulainn was said to be dauntless and bold. When a battle begins, blood sprays from his pores, his jaw swells up to the size of his head, and his mouth splits open from ear to ear, giving him a terrifying appearance and causing his enemies to quake in fear. Cú Chulainn trained in war under the goddess Scáthach, who lives in the Land of Shadows, and earned the magic spear Gáe Bulg. Later, he broke the taboo (geis) upon him and grew weak as a result. Under a terrible curse, he finally met his demise pierced by his own spear. Some suggest that Cú Chulainn was a real historical figure, and today there exists a stone called the "Stone of Cú Chulainn," to which the hero is said to have tied himself in his dying moments."
—Shin Megami Tensei IMAGINE Compendium
Cu Chulainn can only be obtained through fusion. He acts as a boss with Morrigan within Shibuya's Quartz gold level instance and plays a central role as an NPC and enemy in the Foreign County in the Woods series of quests.
He is one of the demons it is possible to obtain as a mount. When the player uses him as a mount, he carries the player bridal style and gazes into the player's eyes, regardless of gender (see the Gallery). His armor appears as equipment for the player along with an all-black variant. It is also possible to acquire his spear, Gae Bolg, which grants the ability to use a penetrate affinity skill by the same name when equipped. Gae Bolg can be obtained by using the care command on the demon or through synthesis.
"A gallant Celtic hero, son of the sun god Lugh. He is said to have beaten an army singlehandedly. He was cursed to die after spurning the war goddess Morrighan, and was impaled on his own mighty Gae Blog."
—Shin Megami Tensei: Strange Journey Compendium
Cu Chulainn appears as an NPC in the EX Mission A formal search request, in which his mother has traveled to the Schwarzwelt to find him. The trail for him leads first to Sector Bootes, where an Orthrus will indicate he has moved to Sector Antlia to offer support to his old teacher Scathach. Meeting with Scathach will reveal Cu Chulainn has heard of the search and is moving to meet the protagonist.
Near the entrance to Sector Antlia, Cu Chulainn will be found with Enemy Search. He will leave for Sector Carina to meet with his mother; once met there, he will reward the protagonist by unlocking Cu Chulainn fusion, giving him Gae Bolg, his spear, and giving him the item Knight's Oath, which contains a Demonica code to summon him.
He appears in the Challenge Quest, Cu Chulainn's Training. His Teacher Scathach has has turned Cu Chulainn back into Setanta and wants Flynn to face him so he learns the way of the warrior. After learning the lesson, she restores Setanta back to his original form and thanks Flynn.
"A gallant Celtic hero who wields the mighty spear, Gae Bolg. He is the son of the sun god Lugh and Deichtine and possessed formidable strength ever since childhood. In battle, his normally beautiful appearance turned fearsome and is said to have turned away an army on its own. He was cursed to die for spurning the war goddess Morrigan's love and was impaled on his own mighty Gae Bolg. Even then, though, he tied his body to a pillar and died without ever falling."
Cu Chulainn appears as the Persona of the Tower Arcana in Persona 3. He can be summoned once the protagonist reaches Lv. 40 or above. Cu Chulainn, along with Scathach also grants the protagonist the Fusion Spell Shadow Hound, which inflicts severe Strike damage upon all enemies.
Cu Chulainn returns as the Persona of the Tower Arcana in Persona 3: FES, retaining all of his attributes. However, as FES introduces the weapon fusion system, Cu Chulainn can be fused with any Nihil item into a distinct weapon at the antique shop via the Weapon Fusion system. The weapon is Gae Bolg (A spear given to Cu Chulainn by Scathach), a Spear holding 260 ATK, 85 Accuracy, and the passive ability Wind Amp. It should also be noted that Gae Bolg inflicts Wind-based damage upon striking enemies with it, rather than dealing pierce attacks.
Cu Chulainn returns as the Persona of the Tower Arcana. He can be summoned once the protagonist reaches Lv 46 or above. Cu Chulainn's in-game model is also re-used as another Persona of the Sun Arcana, Tam Lin; Though two Personae shares different attributes and color schemes.
In the True Ending Special, Margaret summons Cu Chulainn to attack Yu and help him regain his confidence to fight against Izanami-No-Okami. However, Cu Chulainn was repelled by Suparna of the Sun Arcana, and Arahabaki of the Hermit Arcana.
Cu Chulainn appears as a normal enemy encounter in Digital Devil Saga: Avatar Tuner. He can be easily found in Coordinate 136. Cu Chulainn is also one of the demon allies summoned by the optional boss Demi-fiend during battle; he focuses on lowering the party's Defense and striking them with powerful Physical attacks.
Cu Chulainn returns as a normal enemy encounter in Digital Devil Saga: Avatar Tuner 2. He can be easily found in final dungeon, the Sun. Cu Chulainn often appears alongside with Pallas Athena when fought.
Cu Chulainn is a demon of the Genma race and can be fused once the protagonist reaches level 39. As a unique demon only one Cu Chulainn can be possessed by the party at a time. Cu Chulainn is one of only a handful of unique demons (including Shiva and Norn) to appear in free battles, consistently appearing on day 6 and 7.
"A gallant Celtic hero who wields the mighty spear, Gae Bolg. He is the son of the sun god Lugh and Deichtine and had mighty strength ever since childhood. In battle, his normally beautiful appearance turned fearsome and is said to have singlehandedly turned away an army. He was cursed to die for spurning the war goddess Morrigan's love and was impaled on his own mighty Gae Bolg. Even then, though, he tied his body to a pillar and died without ever falling."
—Devil Survivor 2 Compendium
Cu Chulainn returns using his old design from Soul Hackers. He and Tam Lin no longer share the same, palette-swapped appearance.