Gawain

One of the Knights of the Round Table that appeared in the Arthurian legends, also known as the White Knight of the Round Table.
If King Arthur was the night, the symbol of the moon itself, then Sir Gawain was the day, which shoulders the sun. He acted as King Arthur's impersonator in the royal castle of Camelot. In other words, he was one of the candidates to substitute the King after his fall. He is the bearer of the sister-sword to Excalibur, Excalibur Galatine.

In Fate/Extra, he's summoned by Leonardo B. Harwey as a Saber-class Servant for the Moon Holy Grail War, bein the last Servant that Hakuno Kishinami faces. Later, with their defeat, Leo finally learns of defeat and thanks Gawain for his services; Gawain points out to him that he was honored to witness an honorable king before fading away.

In Fate/Grand Order Camelot, Gawain, along with the other Knights who swore allegiance to the Lion King, slew the remainder of the Round Table (including his little sister Gareth), and was, like the others, given a divine Gift to help him "pacify" the people of the Singularity, that being the Gift of Nightlessness: making the sun shine anywhere he goes even at night ensuring he is at his peak at all times. Gawain is placed in charge of the Holy Gate, the entrance to the White City of Camelot.