Y/n LeFey, the unheard of twin brother of Mordred, what happens when Gudako summons(first time being summoned from the Throne of Heroes by the way) him during the Fuyuki incident, and he helps save Olga Marie(by generating a spirit Origin that makes her a Demi-Spirit). And faces Saber Alter who notices how similar he is to her sister. When Salter realizes who, or rather what Y/n is, she goes on a rampage, terrifying him, but he uses Call for Mother, stopping her and causing her awe over him. As for how he got into the Throne of Heroes, not sure, maybe a special event will reveal that for you.

He is weak bodied, a head shorter then Mordred, but had extremely high magical power. Plus very bashful and shy, so women are usually aweing at him. He was also seen as a pawn to Morgan LeFey, but one with no purpose. The only person he ever trusted and cared about in human life was his sister, Mordred, who is usually seen holding Y/n protectively close.

Rank: 5 star

strength: D
Endurance: D
Agility: B
Magical energy: A+
Luck: B
Noble Phantasm: A+

Noble Phantasm: Azure LeFey(Art) release a powerful beam of magical energy from his staff, which he pulled from a dead tree growing on the edge of the very lake that holds the lady of the lake, similar to Artoria and Mordreds Noble Phantasms, with a azure blue steam like attribute.(generates fire from the hot and tempered emotions of himself and everyone around him and water from the air)(will leave user extremely weak and injured)
"Now, my plea resonates, my anger roars while my sadness cools, collide! And erupt so my mother may notice, may my desire be fulfilled! AZURE LE-FEY!"

Skills: Call for Mother(like when your scared and call for mommy and stuff)(Female opponent will be stunned/"in love" and female allies will have increased attack and swiftness)
Charisma B+
Art enhancement

Historical story: the twin brother of Mordred, born from Morgana LeFey, he was a runt and overlooked because of his sister, Mordred. He had to fend for himself when he could finally walk and talk. He found the lake, the very one the Lady Of the Lake, where he found a dead tree, partially in the water, a single branch glowing azure blue. He grabbed hold of it a ripped it from the tree, its structure changing to that of a wooden staff with azure crystal. 

His staff will one day become a sword, the banner of Caster shall be replaced with that of the Saber, so says the Lady Of The Lake.

Saber version:
Y/n Pendragon
Rank 5 star

Strength: A
Endurance: B
Agility: C
Magical energy: B
Luck: B
Noble Phantasm: A+

Noble Phantasm: EXCALIBUR PHOENIX(Buster)(a concentrated beam of pure heated magic formed from the reborn state of Y/n, a true embodiment of a phoenix if there was one)(will allow the user to revive after defeat 3 turns after use(applys guts basically)

Scenario 2: in a past life the reader was the twin brother of Mordred, but instead of going to the throne of heroes, hes merely reborn into the modern era, and is one of the masters called to Chaldea.