He is a class Saber servant, but due to the immediate reveal of his true name in his first appearance, is the story he is referred to as Gawain. His Master is Leo B. Harwey.
Gawain has a well-proportioned tall and sturdy body. He is cast as a calm and sweet handsome man.
He considers selfless devotion to be of the utmost importance, and he has dedicated his sword to Leo, whom he has recognized as a true king. When Leo is talking Gawain never tries to express his own views, and he only speaks as a representative of his Master’s opinion.
His voice and tone are clear and refreshing, and he would never insult his enemy even when they are down. Even if his opponent is lacking in power or ability, Gawain courteously engages with them out of consideration for their desire for battle and their determination.

Sir Gawain appears in The Legend of King Arthur, and is a Knight of the Round Table. He is King Arthur’s nephew, and appears alongside Lancelot as one of King Arthur’s most trusted knights.
In the legend Gawain ends up being unable to save the King, however his loyalty was true to the end.
His regrets remain after being summoned as a Servant, so he is determined that this time he will be of assistance to those who hold sway over the fate of the King, and he has sworn to give it his all with every swing of his sword.
Luckily, the Master to whom Gawain has pledged his sword to seems to be very suitable for him.
Gawain believes that through birthright Leo possess the right to become king, and that it is his role protect and nurture the development of the young king.
…at least at first. Gawain eventually learns that Leo has never experienced defeat, and as the people’s king that it is something that Leo will eventually encounter.
No. It’s more that from his time as a Knight of the Round Table he was thoroughly taught the lesson that without experiencing defeat, a person can never become a true ruler of the people.
This upstanding and honest knight doesn’t believe in Leo unconditionally as Julius does, but has sworn to be Leo’s sword until the inevitable time comes when Leo will become a true ruler.
“—Of course. Until that time comes, I will continue to be victorious.”
As Gawain discusses Leo like this, it’s as if an older brother is watching over his younger brother’s growth.
Even to the end, Sir Gawain never tells Leo what “that time” is. Leo, trusting in Gawain does not try to discover what “that time” refers to, and together they continue their streak of victories in the Holy Grail War.

Fate/EXTRA Material: Encyclopedia of Fate/EXTRA

A Saber-class Servant in Nero’s army.
Gawain appears in the legends of King Arthur as an illustrious knight, rivaled only by “Lancelot of the Lake.” The eldest child of King Lot, the ruler of Orkney and Lothian, he is also the nephew of King Arthur, which is why he was occasionally entrusted with the Divine Sword Excalibur. He is known as a virtuous man, and his valor in battle is noted in many stories.
He fought the Main Character as the Servant of Leonardo B. Harwey in Fate/EXTRA.
His belief that he should serve under a King has not changed, and he has chosen to work for Nero since she possesses the Royal Authority.
Despite the fact that he assists her, he has not pledged his loyalty to her. He trusts the Main Character, who helped him to grow further by defeating Leo.
The discovery that Tamamo also has a Regalia shook him, but he was able to assuage his misgivings by thinking, “I can trust Saber more than I can trust that Caster. In the end, I made the right decision.”
Such is Gawain’s nature. He wields his sword with renewed enthusiasm after he finds out that the Main Character was the one who was chosen by the Regalia.
His side story has him facing off with a mysterious intruder, the blue-clad Saber. Sometimes he deserves to show off his cool side!

Fate/EXTELLA material: Encyclopedia of Fate EXTELLA