The Child of Light, a hero from the Ulster CycleWP of Irish mythology. He was a demigod, the offspring of a mortal woman, Deichtine, the younger sister of King Conchobar mac Nessa, and the deity, Lugh, the god who governed over the sun and a member of the Tuatha Dé Danann, the gods of Irish mythology. Deichtine, the daughter of a druid, Conchobar's father, and Maga, the widow of Ross the Red, disappeared in the Land of the Young without marrying anyone. There, she bore Sétanta, the child of the sun god.
A member of the Knights of the Red Branch and the strongest warrior in Ulster, at the same time, that he is a hero that employs the magic spear received from the leader of the underworld Scáthach, he is also said to be a practitioner of runes - the magecraft of Northern Europe that was inherited from his mentor.
The Norse God Odin, having foreseen the plans of both Goetia and the Alien God, used Cú Chulainn as a proxy to help him save the Human Order. Cú Chulainn was initially summoned to the Fuyuki Singularity as Lancer, but Odin granted him his Authority, transforming him into Caster and giving him some of Odin's abilities.
As a Caster, he does not put on any metallic equipment.
Having manifested as a Caster, he apparently has imposed upon himself the role as one who guides. Not as a true druid, but as a temporary one, as long as he continues together with his Master, he shall illuminate the path that the Master walks on.
Though he is a Pseudo-Servant of Odin, Cú Chulainn is the persona in control of the body.
- Lancer (Cu Chulainn)
- Caster feels miserable whenever Lancer mocks him for being a "spearless me."
- Caster has a tendency to make remarks about Mash's body, complimenting it and saying the view of her behind "is nice."
Fuyuki: Flame Contaminated City
When Fuyuki turned into a Singularity, the city became engulfed in flames, the humans vanished, and only the Servants remained. Artoria Alter restarted the Holy Grail, and proceed to defeat Medusa, Hassan of the Cursed Arm, Musashibou Benkei, EMIYA, and Heracles. Afterwards, they were corrupted by the black mud from the Grail. By the end, the survivors of the Holy Grail War were Cú Chulainn and Artoria Alter. Knowing the war won't stop until Artoria Alter is defeated, Cú Chulainn started his desperate search for allies.
He comes across Ritsuka, Mash, and Olga Marie Animusphere while they're being beset by Hassan of the Cursed Arm and Musashibou Benkei. He helps them kill the two in the hopes of recruiting them as allies against Artoria Alter. He explains to the group the Singularity's circumstance and why he helped them. After forming a temporary contract with Ritsuka, he deduces the Greater Grail is the source of the Singularity, guarded by Artoria Alter, EMIYA, and Heracles. While they can deal with EMIYA if he's with them, Cú Chulainn suggests the group ignore Heracles.
Later, he draws misfortune runes on Olga's coat to attract monsters in order to help Mash unlock her Noble Phantasm. However, even after fighting many monsters, Mash is still unable to use her Noble Phantasm. Cú Chulainn decides to spar with her to help with that. He unleashes Wicker Man, but Mash unleashes her Noble Phantasm to protect the others. However, she still doesn't know its True Name, so what was unleashed was only a psuedo-deployment. Olga dubs it Lord Chaldeas as a temporary name for Mash to activate it.
The group eventually enter the Grail's cavern, where they defeat EMIYA. Afterwards, they prepare to proceed to Grail's chamber. They rest first though as Ritsuka is exhausted from having their Magic Circuits running at full capacitity for the first time since becoming a Master.
Arriving in the Grail's chamber, the group confront Artoria Alter. She then launches Excalibur Morgan to test Mash's Noble Phantasm. After Mash blocks it, the group fight and defeat Artoria Alter. She declares the Grand Order, the battle for the Holy Grail has only just begun before disappearing. Cú Chulainn tries to ask what she meant by that when he notices himself disappearing. He then disappears, asking Ritsuka to summon him as a Lancer next time.
In the First Order movie, Chulainn had already defeated both Hassan of the Cursed Arm and Darius III, before meeting Ritsuka Fujimatu and Mash. He also battles EMIYA alone, managing to beat him in a close battle, and delivers the final attack to Artoria Alter with his Noble Phantasm, Wicker Man.
Event: Garden of Order
Avalon le Fae: Fairy Round Table Domain
Cú Chulainn reappears, calling himself Grímr the WiseWP. When Cú Chulainn returned to the Throne of Heroes after defeating Artoria Alter in the Fuyuki Singularity, Odin let him retain his memories of that summoning. He was then summoned into the English Lostbelt a year before Chaldea arrived there. Cú Chulainn spent the year waiting for Chaldea to arrive learning what he could about Odin's actions in the Lostbelt as Grímr 6,000 years ago, as he was not given that information when summoned.
When Mash loses her memory in the Nameless Forest, Cú Chulainn follows and watches over her in the form of a white wolf. He reveals himself when Morgan's army attacks Sheffield, turning back to his true form to fight off Fairy Knight Lancelot while Mash and Habetrot escape. After the survivors of Sheffield flee the city, Cú Chulainn - once again in his wolf form - accompanies Mash and Habetrot through the Vein Corridor to reach Norwich.
Interlude: Sage of the Forest
Cú Chulainn senses a distortion in a forest in Britannia, but Romani doesn't get any readings. After defeating werebeasts that appeared, Cú Chulainn speaks about his abilities that were given to him as a druid when he was summoned as Caster. The source of the distortion, the ghosts of druids, then appear, but they're soon destroyed by the group. With the distortion gone, Ritsuka asks Cú Chulainn if he wants to visit Ulster, but he refuses as he doesn't want to disturb them with his sudden return.
Having manifested as a Caster Class Servant, Cú Chulainn underwent a sudden change from when manifesting as a Lancer and now torments his opponents as a user of diverse Magecrafts. In this form, Cú is a formidable combatant and is regarded as a powerful Caster by Mash. Prior to the arrival of Chaldea to the Fuyuki Singularity, he defeated Shadow Rider and later on Lancer Medusa or Musashibou Benkei (depending on the adaptation). He has also been shown fighting on par with EMIYA both at short and long-range combat and has even been implied to have fought Saber Alter in the past.
He can transform into a white wolf.
- Territory Creation (B Rank): He can create a territory that simulates Scathach's Gate of Skye. However, he refrains from using it since it might get his childish mentor mad.
- Rune Magecraft (A Rank): Possession of the Magic Crest of Northern Europe - the runes. By making proper use of this, powerful and varied effects are mastered. However, simultaneously employing multiple effects (joint use) is inadvisable. This skill has Ranked UP due manifesting as a Caster. Other than attacks, he mainly uses this to temporarily simulate the effects of other Skills, such as Magic Resistance or Clairvoyance. By employing the "ansuz" rune, Cú is able to perform powerful heat・flame attacks, his specialty. By summoning 9 and making use of his Territory Creation skill, Cú is able to rip off his mentor's spell which summoned the Gate of the Underworld. By doing so he is able to completely nullify Caladbolg II and force Shadow Archer to fight him at close range.
- Protection from Arrows (A-Rank) is the ability to deal with projectiles, an increased defense against ranged attacks by predicting the projectiles' trajectories through exceptional means, such as hearing the sound of air being cut, or sensing the killing intent of the enemy.
- Disengage (C Rank): Bonus effect of returning battle conditions to what they were at the beginning of the match.
Cú's extremely powerful trump card however is his Noble Phantasm Ochd Deug Odin, which activates by invoking all 18 Runes conferred by Scathach together with the release of its True Name. The power of the rune obtained by the great god of Northern Europe is temporarily released to bestow large-scale magic damage on the enemy camp. Furthermore, surviving enemies will have all buff effects dispelled, the parameters of their abilities will be forcibly reduced by 1 Rank and, in case they possess constantly active Noble Phantasms, those will be halted for 1~2 turns. There is also a possibility that use-restrictions have been applied by Odin—but details will be left as unknown for now.
Wicker Man, his second Noble Phantasm, was used to kill Saber Alter. A giant composed of countless tree branches makes his appearance. Said giant wears flames on his body and applies intense heat・flames damage by rushing on the target. The cage on the torso of the giant that appeared as a Noble Phantasm is empty, hence the giant rages while seeking the offerings to the gods. This is not a secret art of the runes, but a Noble Phantasm of the Celtic druids, given to the Child of Light who manifested as a "Celt magus" that manipulates sweltering heat.
He is also known as the Prince of Light (Provisional Caster) (光の皇子（暫定キャスター）, Hikari no Ōji (Zantei Kyasutā)?) in Fate/GUDAGUDA Order.
Mata is the character illustrator for the Caster version of Cú Chulainn.
 Fate/Grand Order - Caster Profile [T]
Cú Chulainn - Caster
Illustrator and Voice actor
Voice Actor: Nobutoshi Canna
Noble Phantasm: B
Rune Magic A
Protection from Arrows A
Territory Creation B
Wickerman: Cage of Scorching Flames
A brave warrior from Celtic, Ulster mythology.
A member of the Knights of the Red Branch and the strongest warrior in Ulster, at the same time that he is a hero that employs the magic spear received from the leader of the underworld Scáthach, he is also said to be a practitioner of runes - the magecraft of Northern Europe that was inherited from his mentor.
Level 1 Bond
Source: Celtic mythology
Alignment: Lawful Neutral Gender: Male
When a Caster, he does not put on any metallic equipment.
Level 2 Bond
Having manifested as a Caster, he underwent a sudden change from when manifesting as a Lancer and now torments his opponents as a user of diverse magecrafts.
Has powerful heat・flame attacks as his specialty. Apparently, the "ansuz" rune is being employed in this.
Level 3 Bond
- Rune Magic: A
Possession of the Magic Crest of Northern Europe - the runes. By making proper use of this, powerful and varied effects are mastered.
However, simultaneously employing multiple effects (joint use) is inadvisable.
Level 4 Bond
"Cage of Scorching Flames"
Rank: B Type: Anti-Army
A giant composed by countless tree branches makes his appearance. Said giant wears flames on his body and applies intense heat・flames damage by rushing on the target.
Level 5 Bond
The cage on the torso of the giant that appeared as a Noble Phantasm is empty, hence the giant rages while seeking the offerings to the gods.
This is not a secret art of the runes, but a Noble Phantasm of the Celtic druids, given to the Child of Light who manifested as a "Celt magus" that manipulates sweltering heat.
Having manifested as a Caster, he apparently has imposed upon himself the role as one who guides.
Not as a true druid, but as a temporary one--
So long they continue together, he shall illuminate the path that the Master walks on.
クー・フーリン - キャスター
- Rune Magic: A
- Fate/Grand Order: Cosmos in the Lostbelt - Avalon le Fae: Fairy Round Table Domain - Chapter 19 Section 3
- Fate/Grand Order - Kara no Kyoukai: The Garden of Order, 7th Floor, Room No.4
- Fate/Grand Order: Cosmos in the Lostbelt - Fairy Realm of the Round Table: Avalon le Fae - Chapter 7 Section 4
- Fate/Grand Order: Cosmos in the Lostbelt - Fairy Round Table Domain: Avalon le Fae - Chapter 7 Section 7
- Fate/Grand Order - Septem: Section 15 God's Whip
- Fate/Grand Order material I & II: section on Jing Ke, Charles-Henri Sanson, Orion and Artemis, Edward Teach, Kiyohime, Asterios, Saint Martha and Marie Antoinette Skills, translated by Master of Chaos at Beast's Lair.