Diarmuid Ua Duibhne

The Lancer class Servant of the Fourth Holy Grail War, summoned by Kayneth El-Melloi Archibald. A hero of the Fianna, spoken of in Irish mythology. The story of his tragic love affair with Grainne would later become the model for the story of Lancelot and Guinevere. It's no wonder that the Berserker of the Fourth Holy Grail War has such bad compatibility with him!
In battle, he wields two Noble Phantasms: the Crimson Rose of Exorcism (Gae Dearg) which temporarily nullifies the effects of magical energy, and the Yellow Rose of Mortality (Gae Buidhe) which inflicts cursed wounds that cannot be healed by any means.
His basic parameters are not particularly high, but he excels at using battle tactics designed to trip up enemies stronger than himself. In particular, he is the natural enemy of those Heroic Spirits which rely on the powers of their Noble Phantasms to fight. Depending on his strategy, it's even possible that he could have won the entire Fourth Holy Grail War (or suffer a humiliating defeat at the hands of Assassin).
Out of all the Servants in the fourth war, he had the second-worst compatibility with his Master. Though, if his very first opponent had been anybody besides Saber, he might have been able to fight in a way more in line with his Master's expectations. For you see, it was Saber's serene beauty and grace which kindled the flames of chivalry within his heart.
Also, while he was summoned as a Lancer this time, and therefore carried the twin demonic spears Gae Dearg and Gae Buidhe, his legends also speak of a pair of demonic swords: Moralltach and Beagalltach. So, there was actually a decent chance he could have been summoned as a Saber. Or rather, that's probably what Kayneth was hoping for…

Fate/Zero material: Fate/Zero Encylopedia