Nobleman and renowed mage at Clock Tower. Excellent magus and a cruel man. He is engaged to Sola-Ui Nuada-Re Sophia-Ri, through an arranged marriage.
He's a magus with ancient heritage and great status. He was born with immense talent and rarely if ever encountered any hurdles in life. He's a cruel man with a selfish personality. Kayneth is a teacher of Weaver and the one who tore his manuscript(which proposed a radical new thesis) apart. In return Weaver stole Kayneth's artifact and participated in the Grail War. Kayneth needed to find a different item and became the master of Lancer. He is engaged to Sola-Ui Nuada-Re, the Spiritual Evocation Department head's daughter through an arranged marriage.
Role in the story:
Kayneth is the most active master at the beginning of the War. While Lancer and Saber fought he hid behind his invisibility spell and observed the battle. Thanks to Lancer's clever tactic, the battle was turning out to be in his favor until Rider's interruption. When he found out that Weaver participate in the War as well, he revealed his identity to his former pupil and made a deadly threat. Soon after the three-way battle transformed into four then five-way with the sudden introduction of Archer and Berserker. In the middle of chaos when Berserker attacked Saber, Kayneth ordered Lancer to attack her as well. He initially refused but Kayneth forced him with the Command Spell. Thanks to Rider's interruption again and Kiritsugu finding Kayneth via infrared binoculars, he had to retreat.
His wife, Sola-Ui called him a coward for retreating. Their argument ended with Lancer standing up to his master. Unknown to both master and Servant, that later became a source of strife as Sola-Ui developed feelings for the Irish hero. Kayneth made his base at Hyatt Hotel. He booked the top 24 floors and transformed it into a magical fortress with various magic barriers, traps, summoned demons. But he didn't expect that someone would just choose to blow up the basement. Kiritsugu did exactly that with a bunch of C4 charges. With the protection of Volumen Hydragyrium Kayneth and his wife just barely managed to survive the building's collapse. In one night he lost all the special equipment he prepared for the War alongside his base.
He moved to an abandoned warehouse and made it his new base. His relation to his wife worsened with every failure. In revenge Kayneth assaulted the Einzberns. He ordered lancer to halt saber while engaged Kiritsugu personally. Volumen Hydragyrium proved to be a very deadly weapon but due to Kayneth's overconfidence and Kiritsugu's special abilities he was defeated. Kiritsugu hit him with his Origin Bullet, nearly killing the mage. Lancer quickly went to his master's aid and that saved Kayneth. The genius mage was rendered helpless and Sola-Ui abused the situation to blackmail him. In exchange for healing him, Sola-Ui got the ownership of Lancer.
Kayneth realized his wife's secret feelings for Lancer but he was helpless to do anything about it. He became a cripple that couldn't even use magic anymore. The loneliness, frustration and jealousy almost made Kayneth mad. He called the famous puppet-maker in secret. With first-class magical protesis, Kayneth could finally move once again. After Lancer in conjunction with Rider and Saber defeated Caster, Kayneth went to the Risei( the supervisor) to claim his reward. As he got what he wanted, Kayneth shot Risei out of cold blood to prevent any other masters to get additional Command Spells.
He quickly re-estabilished his contract with lancer and felt confident again. Meanwhile Kiritsugu kidnapped Sola-Ui. With some inspection Kiritsugu found the mage's hideout. Saber confronted Lancer once again while Kiritsugu met up with Kayneth. In exchange for Sola-Ui and his safety, Kiritsugu asked the mage to order Lancer to suicide. They made a mage's contract, an unbreakable oath. Kayneth gladly accepted the conditions, thinking that he can live with Sola-Ui in peace. With the power of Command Spell, Kayneth asked lancer to kill himself. Lancer couldn't do anything but curse as he pierced his own heart. But the contract had a loophole. Kiritsugu only swore to not kill them personally.It said nothing about his assistant, Maya. From distance Maya shot down both kayneth and Sola-Ui, gravely injuring the mage and instantly killing his wife. The dying Kayneth asked for quick death, but Kiritsugu refused it due to the contract. Seeing that Saber finally put Kayneth out of his misery.
Kayneth was arguably the best magus in the 4th Grail War. His prodigal talent was only matched with his overconfidence and pride in his superiority. His elemental affinities were wind and water and were proficient with summoning, alchemy and spiritual evocation. Kiritsugu ranked him as one of the most dangerous threats in the War(right after Kotomine).
He made his base at top 24 floors at Hyatt Hotel and transformed it into an impenetrable fortress, with various Boundary Fields, traps and even summoned monsters. He also made a spatial alteration at the main lobby to confuse the intruders. Unfortunately though by a miscalculation he lost his base and much of his equipment. Despite his loss he still remained a formidable foe until the end.
Some magics worth to mention:
Healing: Kayneth can heal moderate wounds in seconds. The magic can repair the damaged clothes as well and essentially looks like he turned back time.
Invisibility: Kayneth initially used this to masks his presence while he controlled Lancer. Through an illusion he became invisible to the naked eye. His voice became untraceable as well. In the end Kiritsugu used infrared binoculars to find him.
Volumen Hydragyrum: Kayneth's only remaining Mystic Code(magic item) after he lost his first base at Hyatt Hotel. Volumen Hydragyrum is actually a magically enchanted large puddle of mercury. It's overall weight sums up to 140 kg(roughly 10 liters of volume) and Kayneth can use it for both offense, defense and even reconnaissance. He could alter it into a whip to cut even solid steel or form it into a spherical barrier to fend off physical attacks. Amongst others Volumen Hydragyrum withstood explosions, weight of several stories of rubble, claymore mines and continuous automatic fire. For tracking it could extend countless thin appendices. In this "hunting mode" the appendices sense any form of vibration, change in atmospheric pressure or sound, even heartbeat. Volumen Hydragyrum is the best Mystic Code of Kayneth and the one he has the most expertise with.