Saber's True Name is Charlemagne (シャルルマーニュ, Sharurumānyu?), Holy Roman Emperor, King of The Franks, leader and namesake of the Twelve Paladins of Charlemagne. The legend of Charlemagne and his paladins is based off of the real life history of Karl der Große. They are both the same Heroic Spirit with the same Saint Graph, but Charlemagne would normally never be summoned, due to the real exploits of Karl as a king and emperor far outweighing the fantasy of Charles as a hero of chivalric epics. He is only summoned due to the specific events of the story, but at the end of Extella/Link Altera proposes a way to register a unique Saint Graph for him, allowing him and Karl to be summoned individually.
Charlemagne appears as a young man with black and white hair and blue eyes. His attire consists mainly of a black and white tunic with blue accents, black pants, thigh-high armored boots, and a short white cape with a black edge. Aside from his boots, his left arm has a gauntlet, and he wears a set of faulds at his waist. A sword scabbard hangs at his left side, and red marks coat his right gloved arm. He bears a resemblance to the historical Charlemagne during his youth, (more specifically when he first became a knight at and received his legendary sword Joyeuse), but has black/white hair in place of his historically recorded brown or blond hair.
A carefree young man who somewhat bluntly says that he views himself as not very suitable to be King of The Franks or Holy Roman Emperor. He bases his aesthetic standard and behaviour on whether something is "cool/awesome" (かっこいい, kakkoii) or not, even if that will be a handicap for the Master, or even to himself. Despite this personality flaw, Astolfo describes Charlemagne as a conscientiously benevolent king that would weep and laugh with his people and also help those who are in need.
Charlemagne prefers for people calls him Charlie, Chuck, or Charles and often insists his comrades to call him by the shortened version of his real name, as he considers "Charlemagne" to be "kind of a heavy name for me to carry around these days". This may be an attempt to differentiate himself from Karl der Große.
He also expresses initial disapproval of Astolfo's crossdressing, despite Astolfo's insistence in justifying this habit. Charlemagne says that such a hobby would make "Daddy" cry, much to Astolfo's chagrin. He later expresses surprise to Astolfo "that [you] look so good in those".
When facing Altera, Charlemagne addresses her as "Sister" (姐さん, Nee-san?).
When Hakuno's allies discover that Karl der Große is the enemy, they question Charlemagne by asking if he and Karl are considered the same person. Charlemagne simplifies things by saying that his situation is similar to Li Shuwen, or the Alter Servants, where Charlemagne is derived from the "golden age" of Charles the Great as a swordsman and Karl der Große is the "golden age" of Charles the Great as a king; both are the "same person" but have contradicting personalities.
Because Charlemagne only exists as a "shadow" of Karl, he either disappears from SE.RA.PH or is implied to have done so in all three routes following Karl's defeat. However, at the end of the third route, Altera proposes a potential method to make the Moon Cell register distinct Saint Graphs for him and Karl, by writing new life histories for each of the two men. This process would normally take a lifetime, but Altera suggests that they speed up the process by summoning them as Attack Programs and having them fight new battles in SE.RA.PH.
Along with his main weapon, the sacred sword Joyeuse, Charlemagne is shown to be able to use the weapons of the Paladins due to his Holy Knight Emperor skill, like a serrated long sword, a halberd, a mace, Trap of Argalia, and has six red and six blue winged, javelin-like weapons floating behind him, that unleash multiple colored blasts.
Holy Knight Emperor (聖騎士帝, Seikishitei?) is a title skill bestowed to Charlemagne as the one who leads the holy knights (paladins) known as Twelve Peers. It provides a tremendous Charisma, improved attacks against the demonic, improved defences against Divinity, aptitude for weapons of the holy attribute and others. Also, this skill makes possible to invoke the Noble Phantasm, Joyeuse Ordre.
Mana Burst (Light) (魔力放出（光）, Maryoku Hōshutsu (Hikari)?) is a subspecies of Mana Burst that is allowed only to saints, holy maidens, and holy knights. It endows Charlemagne with an advantage against the evil aligned.
Over the Royal Road (王道踏破, Ōdō Tōha?) is a self-admonition type skill that displays its effects by having the user stick to the attitude of wanting to be a king. It contributes to the improvement of a portion of their parameters, but weakens them upon taking a conduct that violates that kingship. In Charlemagne's case, this means that he "gets weakened when doing something lame" and that "some sort of gauge rises when doing things that he finds cool".