The Servant's True Names are Dioscuri Castor (ディオスクーロイ ・ カストル, Diosukūroi Kasutoru?) and Dioscuri Pollux (ディオスクーロイ ・ ポルクス, Diosukūroi Porukusu?), two siblings from Greek mythology. Castor was trained by Chiron, noted among fellow "shining stars" students like Heracles, Achilles, Jason, and Asclepius. He was sublimated as the Gemini constellation.
- ↑ Fate/Apocrypha volume 2
 Fate/Apocrypha material - Encyclopedia: Archer of “Black” [Servant], p.159-160 [T]
Archer of “Black” [Servant]
One of the Servants of the Black camp. His true name is Chiron. He is a centaur—a half-human-half-horse race, and he is a famous great sage in Greek myth.
He has a calm and gentle personality, but though he treats his Master with respect, he won’t hold back on advice if it’s for the sake of his Master. He never scorns or despises others regardless of whether they are enemy or ally, and he is exceptionally calm among the intense and passionate heroes of Greek myth. He acted as the strategist of the Black camp and continued to support his camp day and night right until he took part in the final battle.
Usually, centaurs are considered both as great huntsmen who effortlessly use bows and arrows while running with their horse legs and as savage monsters who would steal anything and everything. It seems Chiron alone is regarded as a “sage” that stands out as an exception among them.
Chiron was born from Cronus, father of the head god Zeus, and Philyra, the goddess of a certain island. However, because Cronus had copulated with her while she was turned her into a horse, Chiron was born as a half-human-half-horse centaur. (There is also the theory that they were attacked by Cronus’ wife Rhea while they were in the middle of copulating and Philyra turned into a stallion in order to run away.) Balking at the idea of giving milk to a monster like Chiron, Philyra changed her form into that of a linden tree.
Though he was never loved by his mother and father, Chiron became a sage that excelled in all forms of knowledge as he grew up. This is not necessarily unrelated to the fact that his mother’s name “Philyra” means linden tree. The leaves of linden trees can be used as restoratives, and the tree bark can be used in divination or as written boards.
After becoming an adult, Chiron began to rear the “future heroes” who implored for his tutelage from all over Greece. Besides Heracles and Achilles, the heroes who received his teaching include those like Asclepius, who later became the god of medicine, and Castor, who was sublimated as the Gemini constellation. Jason, the leader of the Argonauts who enticed Medea, is also one of his students.
On a certain day, someone shot a poison arrow at him while trying to stop a dispute between centaurs, and the poison continued to endlessly make him suffer due to his immortality, until he became the Sagittarius constellation by returning his immortality to the gods and finally received salvation. His wish for the Holy Grail while participating in the Holy Grail War is to regain his immortality. To him, it is the only gift he received from his parents.
As a Servant, his stats have received a slight rank down due to giving up his immortality and transforming his legs into those of a human, but even then he boasts of specks high enough for him to be called a first-rate Servant—to an extremely abnormal extent in a certain sense. In the first place, it appears that he taught not only swordsmanship, archery and horse-riding, but also wrestling techniques, so he is relatively experienced in moving with human feet.
With the combined effects of his skills, he can even utilize a pseudo-form of future sight, making him truly all-purpose in his abilities. If he has a weakness, it is that he lacks a decisive trump card. Against the brute force approach that throws away all wisdom or intelligence, his strong points fade away.
He deeply loves how weak and insufficient humans struggle despite their insufficiency and still try to move forward, so even if he was summoned as part of the Red camp, he would have definitely rebelled against them with the resolve to die.
Though if he were to describe it himself, he would merely answer, “This is my arrogance as a teacher, as embarrassing as it is”.
In the novels, he was mainly focused on his fight with Achilles outside of his light skirmish with Mordred. After all, Chiron was the only one capable of harming Achilles. That battle was such a great joy to him that he even forgot his true nature as a Servant, but he stood back up as a Servant again at the moment of his death and accurately dealt a fatal wound to Achilles.