Also known as: アーサー・オルタ
Saber-type Servant that was first mentioned in an art book containing promotional material for Fate/Prototype. He is the Alter version of the Prototype universe's Arthur Pendragon (Fate).
Notably, Arthur Alter's Excalibur (Fate/Prototype) is said to be untarnished, unlike Saber Alter's Excalibur Morgan (Fate). Despite this, fandom interpretation of Arthur Alter often has him use a corrupted Excalibur similar to Saber Alter's own.
Visually, he shares Saber Alter's Yellow Eyes and platinum Blonde Hair, and wears black Armor.