Vivian

Vivian was a Great Mother-class fairy born in the Inner Sea of the Planet. Also known as the "Fairy Princess of the Lake," Morgan le Fay was considered her counterpart, Vivian embodying goodness compared to Morgan's evil. This duality was also reflected in the nature of Excalibur itself. It was believed by some that Merlin himself taught Vivian magecraft, and they also had a love affair.

Vivian granted Artoria Pendragon Excalibur and Rhongomyniad after the loss of Caliburn. Vivian also became her guardian. Near the end of Artoria's reign, when Merlin fled to Avalon to escape Vivian after scorning her, Vivian imprisoned him in a tower where he remains even now.

(Source: TYPE-MOON Wiki)