In the 4th Holy Grail War, Lancer serves as the loyal servant of Kayneth Archibald El-Melloi. He is known for his chivalry and sense of honor, in contrast to Kayneth's arrogant personality. Although they are rivals, Lancer wishes to face Saber in fair combat. Lancer was summoned by Kayneth as an Rider-class Servant, but because Waver Velvet stole the artifact meant for use as a catalyst, Diarmuid Ua Duibhne was called instead. Unlike other servants, Lancer's contract with Kayneth is unique in that Sola-Ui maintains the energy to materialize him while Kayneth holds the Command Spells.
His true identity is Diarmuid Ua Duibhne, the son of Donn, foster son of Aengus Óg and a warrior of the Fianna. Known as Diarmuid of the Love Spot, the appellation resulted from an encounter with Gráinne, a woman who caused a magical love spot to appear on his head. Any woman who looked upon his face would instantly fall in love with him.