Caster (Fate/Grand Order - Merlin)

Merlin (マーリン) is the court magus of King Arthur. He is also called the Magus of Flowers (花の魔術師).

Background
Mārin is the son of the Queen of Wales and an incubus. He is an incubus hybrid who served King. Uther loved Igraine, the wife of Duke Cornwall, so he secured Merlin's assistance in winning her love at the price of leaving his child in Merlin's care. Merlin told Uther a prophecy about his child, who would become his appointed successor during their war-torn times. Under an arrangement from Uther, he helped produce an heir bearing the "essence of a dragon", so Artoria was born as a human infused with a dragon's magical energy. Although the child was a girl who could not officially become the successor, Merlin was delighted all the same, believing that her sex never mattered. Confident that her being separated from the castle until the prophecy was fulfilled was proof that she would become king, he entrusted her to the king's vassal, Sir Ector, and had her raised as the child of a knight. Merlin visited at some point and swore Sir Kay to secrecy about the matter of her gender.

He prepared Caliburn for the selection of the next king, but none could pull it out of the stone. As the other participants prepared a jousting match to decide the kingship, Artoria neared the sword and prepared to pull it without hesitation. Merlin appeared before her then, telling her she should think it through and describing that to become "king" is to no longer be "human." Warning her that she should stop because she is fated to die a miserable death while resented by all humanity, he showed her the end she would face. Having prepared for that day since her birth and firmly believing in the people smiling she saw in Merlin's vision, she pulled it without hesitation and became king. As he left her, he said that in exchange for the miracle that she will lose that which is most important to her.

Due to Artoria having issues with discussing her gender and producing an heir, his magecraft was to solve it by making her a pseudo-male for an unknown duration of time, which allowed for the creation of Mordred from her sperm. Artoria lost Caliburn at a later point and obtained Excalibur and Avalon from the Lady of the Lake. It is said in legend that he asked Artoria about which implement was better, chiding her for choosing the sword because the sheath will protect her.

Merlin has left King Arthur before the last battle with Mordred due to his love affairs problem. He claims that the cold-mannered woman has been trying to murder him so he has taken refuge to the other side of the World where he'd crossed the boundary and stumbled into a foreign land known as Avalon. Merlin had thought that the evil witch could not reach him anymore. But he encountered a gate that was built with rough stones, resembling the stone circles. The gate was inscribed with "Only the innocent may pass". Merlin realized that the gate was a trap and he was imprisoned when he had walked into it. Massive walls of stone emerged from the ground in front of him. The walls seem to vertically extend forever, effectively trapping the visiting magus inside. The place is like a tower without a roof.

Merlin is fated to be trapped in this prison, called the Garden of Avalon, for eternity, having transcended death. This also means that there is no possibility of him becoming a Heroic Spirit even if he is worshiped by the masses, since he is unable to die in order to have his soul recorded in the Throne of Heroes.

Appearance
Merlin wears a robe woven with the finest fabrics, though he still looks modest, his long hair appearing in colours of the rainbow under sunlight.

Personality
Merlin has the bearing of someone unlike a magus, much to the point where it would be hard to believe him when he claimed to be one. Noted to be a "troublesome adult", he is spaced out and clumsy, freely enjoying his pastimes and playing around like a child. More immature than Artoria, he was somebody she "couldn't leave alone", but she respected him greatly as a magus. Despite that, she describes him as the cause of every trouble and mentions that the age would have been better off had he been less mischievous. Agreeing with Shirou Emiya that he could be described as a "real bad guy", she describes his worst trait as being "weak against love affairs", or more aptly "full of love." Although it led to his downfall, she believes he is "still casually talking about love."

Despite being a hybrid being, Merlin has never hated humans. Rather, he is fond of them to such a degree that it is beyond natural.

Fate/stay night
In Fate route, Merlin was mentioned when Shirou Emiya was dreaming about Saber's past. Rin Tohsaka briefly mentions Merlin when they discuss about King Arthur legend.

In the Unlimited Blade Works route, Saber describes him as being still active in present tense even when she is summoned as a Servant. Saber reveals that she shares a common trait with Shirou as they were fond of an immature magus. Saber describes Merlin to Shirou.

In the Last Episode, Merlin explains to Saber that her dream to reunite Shirou is difficult and near impossible. He informs Saber that two miracles must occur, in order for them to meet again as normal people. One must wait continuously, while the other must pursue endlessly. They must realize that it is impossible to succeed yet at the same time be capable of enduring patiently.

Garden of Avalon
Half of the story is narrated by Merlin as he was imprisoned in the stone tower, while the other half resembles conversation between Merlin and various Knights of the Round Table. The story tells the Fate version of King Arthur's legend from the day she pulls out Caliburn to her demise.

E Pluribus Unum: Grand Battle of Legends in North America
Merlin makes a brief appearance during the Grand Order in America, wandering through as Cu Chulainn (Alter) prepares to destroy the Protagonist. Remarking that he cannot truly intervene due to daydreaming, he declares that he shall meet the Protagonist properly at the designated future, and requests they take care of his Familiar, Cath Palug, for him.

Abilities
Merlin is a formidable magus, said to be the most feared in the country, who is also called a magician. Bolstered by his Incubus heritage, Rin Tohsaka describes that being able to disguise Artoria's gender or "arrange for a child that should never have been born" should be extremely easy tasks. The creation of Invisible Air is attributed to something he could have thought up for Artoria.

He is a half-blood born from the union of a human and an incubus, he possesses the proof of the Highest Mage; Clairvoyance "the Eyes Which See through the World". Eyes able to see beyond while the person stands in place. From time immemorial, it is the power entrusted to lands by the gods and the power shamans needed to guard lives. However profound and capable of performing the strongest forms of Magecraft one's magic circuits may be, a mage who does not possess these "Eyes" shall not be given the title of the most supreme.

He is shown in possession of, and capable of wielding, Excalibur.