A talented mage and the second child of the House of Archibald. Her older brother was chosen to succeed as the family head and was sent to the prestigious Clock Tower to study; whereas she was self-taught and treated as nothing more than a source of mana for her family. She stumbled upon an old family necklace which she wears (Later becoming the object that she summons her servant from).
Even after all their preparation the grail chooses Rhiannon rather than her brother. Furious and insisting they would lose, they try to force her to give up her crest and command spells. While they begin to steal her mana, somehow she summons her servant, a Lancer who has a history with Kayneth El-Melloi Archibald. Not taking kindly to his new master’s mistreatment, Lancer decides to make them pay before helping her escape the estate.
Diarmuid Ua Duibhne
The First of the Knights of Fianna, Lancer possesses a love spot on his face, which causes females with insufficient magic resistance to fall in love with him should they gaze upon it. As a knight, he holds a strong sense of honor in battle and loyalty to his master, being summoned to fight in the grail war because he wants to display the loyalty to his lord that he was not able to show when he was alive.
The last time he was summoned, during a one-on-one duel with Saber Arturia, Lancer is forced to commit suicide by Kayneth and dies cursing his master and the Grail for destroying his pride and last remaining wish.
Lancer possesses two Noble Phantasms: Gae Buidhe: The Golden Rose of Mortality (Gei Bō), a short yellow spear which inflicts wounds that do not heal as long as the spear exists in the world, and Gae Dearg: The Crimson Rose of Exorcism (Gei Jarugu), a long red spear which cancels mana upon contact, ignoring all magical defenses and temporarily dispelling all magical attributes of the target struck.