Lancer-class servant in the fourth Holy Grail War. Servant of Kayneth El-Melloi Archibald, who is a massive prick and I hate him so much. Literally cursed to make women fall in love with him. He has a tough life, both times he lives it. Fiercely loyal and bound by chivalry. He's so loyal he ends up dying horribly.

Diarmuid is a figure from ancient Irish mythology. He was the first warrior of the knights of Fianna. He was cursed with a magical love spot, causing any woman who gazes upon him to instantly fall in love. This led to Grainne, the bride of Diarmuid’s lord, Fionn mac Cumhaill, falling in love with him at her own wedding party. Grainne placed a magical binding contract upon Diarmuid, to run away with her. Even during their flight and long time together, it is said that he never laid a finger on her out of his loyalty to Fionn. The story goes that Diarmuid single-handedly dispatched a thousand soldiers of Fionn who pursued him. Eventually, after the lives of many of his soldiers were cost, Fionn decided to recognise the marriage of Diarmuid and Grainne, welcoming them back into his land. For a time, they lived happily together. Sometime later, Diarmuid was fatally injured by a demon boar while hunting. Luckily, they were hunting near a magical spring whose waters had restorative healing properties. Fionn, who still held jealousy and hatred in his heart, spilled the water twice while bringing it to Diarmuid. By the third time, he was too late, and Diarmuid had died. The tragic story of Diarmuid and Grainne is the model upon which the story of Lancelot and Guinevere is based. For Lancer, the Holy Grail War is a miracle that he hopes will allow him to regain his honour and pride as a knight by fighting noble battles and ultimately presenting the Holy Grail to his master.

The relationship between Lancer and his master is fragile at best. Kayneth was engaged to Sola-Ui Nuada-Re Sophia-Ri, the daughter of a very prestigious and well-known mage. The marriage had been arranged in order to carry on both family’s legacies. While Kayneth seems satisfied with this arrangement, Sola, on the other hand, is less than pleased. She comes across as cold, heartless, and devious, expressing no affection towards Lord Kayneth. However, the tension between them drastically increases upon the summoning of his servant.

Sola immediately falls in love with Lancer upon seeing him. Her love for him quickly grows into obsession. Kayneth realises this and grows increasingly critical of Lancer, telling him he is a useless servant, being convinced that Lancer will steal his wife from him. Sola springs to Lancer’s defence, calling Kayneth a coward. However, Lancer defends his master, regardless of how poorly Kayneth treats him. In addition to being a genuinely loyal person at heart, he also hopes that his loyalty to his master in this life will make up for his failures in his past life. He demonstrates extreme loyalty towards Kayneth, taking his master’s harsh words silently, never once criticising him. After Kayneth’s magical circuits have been permanently destroyed by an opposing master, Sola blackmails and threatens Kayneth into giving her the Command Seals to control Lancer. By this point, Sola’s obsession with Lancer has reached near-insanity. She begs him to let her be his master, but Lancer, steadfastly loyal to Kayneth, refuses. Lancer, as a knight, closely follows the code of chivalry. He is honourable, proud, noble, and fair.