Lancer served as Kotomine's Servant during the fifth Holy Grail War. He was initially Bazett's Servant but Kotomine killed her and took command of Lancer. Lancer is friendly, laid-back, and chivalrous, but incredibly fierce in battle.
As the Lancer class, Cu Chulainn is one of the speediest Servants among the 7. He is also incredibly durable and good at escaping an unfavorable battle. Cu Chulainn can also be summoned as the Berserker class due to his nature in combat and the Caster class due to his skill with Rune Magic.
Cu Chulainn is the great hero of Ireland. He is a half-god, half-human hero born of the light god Lugh and Deichtine, the sister of Conchobar mac Nessa of Ulster, who appears in the legends of Ulster. His childhood name was Setanta, and it was prophesied from a young age that "this child will live as a hero".
Cu Chulainn means "Hound of Culann". The name comes from an oath Setanta made when he was a child, when he mistakenly killed a guard dog of a wealthy merchant named Culann. He served as Culann's guard while another guard dog is reared.
Cu Chulainn grew up to be a strong man and went into an evil land called the Country of Shadows. He was well liked by the ruler of that country, the witch Scathach, and he trained under her and learned many sorceries and techniques. He obtained the cursed lance Gae Bolg that later became the symbol of Cu Chulainn.
In the legend, it is said that Cu Chulainn was brave, compassionate, and a likeable young man. Celtic warriors had the custom of making an oath called a "geas", and those warriors broke this geas were cursed. This geas later became the cause of his death. During a battle with an opposing country, to protect his weaker country of Ulster, Cu Chulainn made several geasa accepting even his own ruin in exchange for forcing the opposing country to make geasa unfavorable to them.
The leader of the opposing country, Queen Maeve conducted several schemes that forced Cu Chulainn to break his geasa, causing him to lose all his powers and finally killed. Cu Chulainn will not allow himself to die lying down and tied himself to a post.
Morrigan, the goddess of death and destruction was impressed by Cu Chulainn's chivalry offered to protect him, which Cu Chulainn declined. Morrigan was offended and transformed into an animal three times to interfere with his battles but Cu Chulainn injured her each time. Cu Chulainn later accidentally healed her wounds. Morrigan admired his chivalry and watched over him until the end of his life.
Lancer first appeared to attack Rin when she tried to dispel the boundary field in her school. Rin's Servant Archer dueled against Lancer until he suddenly noticed Shirou's presence. Lancer then hunted him down and stabbed him in the heart in accordance to the rules of the war to not allow any witnesses of the Holy Grail War.
Later that night, Lancer hunted Shirou down again to his home when he summoned Saber who dueled Lancer. He used his Noble Phantasm, Gae Bolg to severely damage Saber which revealed his identity. He then retreated, stating that his Master only wants him to scout all the Masters of the war.
Late on the Fate route, Shirou visited the church to seek advise from Kotomine regarding Gilgamesh. There, Shirou discovered that Kotomine was the Master of both Lancer and Gilgamesh. Shirou was then captured and Saber came to his rescue. Lancer did not agree with Kotomine's actions and decided to rebel against him. He allowed Shirou and Saber to escape but was killed by Gilgamesh.
Unlimited Blade Works
After Caster took over the church and ousted Kotomine, he sent Lancer to protect Rin and ally with Shirou and Rin. He expressed his interest in Rin. The next day, Lancer dueled Archer when they assaulted the church. Lancer defeated him but chose not to end him. After Rin was kidnapped by Archer, Lancer followed Shirou and Saber to rescue her at Einzbern castle. Lancer had the knack of pushing Saber's buttons.
At the castle, Lancer found Rin using his searching Rune. There, Kotomine ordered him to kill Rin but he refused. Kotomine then consumed a Command Spell to order Lancer to kill himself. Lancer managed to hold on long enough to kill Kotomine, injured Shinji, and freed Rin. He then used his Rune to set fire to the castle.
Kotomine sent Lancer to investigate Ryudou Temple after Caster was killed. There, he fought against True Assassin but was devoured by the black shadow.
Cu Chulainn's signature weapon. A cursed red lance that inflicts wounds that cannot heal as long as the weapon exists in the world.
Its special ability is to guide itself to pierce its target's heart.
Gae Bolg can be used with 2 techniques which are counted as 2 separate Noble Phantasms.
Piercing Lance of Death Thorns
The first technique is to thrust the lance. Gae Bolg reverses the law of cause and effect; it would have already pierced the heart before the lance is thrust. This technique consumes very little magical energy and it guarantees a kill unless the target's luck is very high.
Saber managed to survive this Noble Phantasm in the Prologue.
Thrusting Lance of Death Flight
This technique unleashes the full power of Gae Bolg. The throw embodies an enormous amount of magical energy that is aimed at the heart.
This attack was used against Archer on the Unlimited Blade Works route. Archer used a 7-petaled Rho Aias to greatly reduce its power before getting hit. Archer still received enough damage to get knocked out without getting killed.