The Saber (セイバー, Seibā?) Class Card of the Fifth Ainsworth Holy Grail War was granted to Zachary Ainsworth. It was one of eight Class Cards transported to Illya's world in Fate/kaleid liner PRISMA☆ILLYA.
The True Name of the Heroic Spirit of the Class Card is King Arthur, Artoria Pendragon, the legendary King of Knights of Britain who pulled the sword of selection from the stone. As there are many sides to her in terms of personality, she normally possesses a "serious personality as straight as an arrow", but Illya's Installation of the Class Card brings out the Princess Knight side of her.
When manifested, Saber takes the appearance of Saber Alter.
The Saber Class Card manifests in the same location as the Caster Class Card, under the Fuyuki bridge. Saber appears after the retrieval of Caster. She overwhelms Illya and Miyu, and even Rin and Luvia utilizing the Kaleidosticks are only able to damage her enough that her barrier is destroyed. She then utilizes Excalibur to easily defeat them. Chloe, channeled from within Illya, then installs the Archer Class Card, and she manages to destroy Saber with a projected version of Excalibur.
Miyu later Installs the Class Card to fight Berserker, but proves to be insufficient to fully defeat him. Both she and Illya later Kaleid Install it to finish him off.
Saber is used by Miyu again when she fights Chloe when Miyu expressed her resolved how she wouldn't care if Illya rejected her and that she will still protect her friend.
In 3rei where Illya and her friends are sent to another world to rescue Miyu, the card is then finally Installed by Illya during her fight with Chloe when Ilya complains how she want to save Miyu but was in turmoil over the overwhelming odds. They both clashed with Excalibur with Illya being the victor.
Illya attempts to use it against Darius Ainsworth , but was proven ineffective.
In the final battle to rescue Miyu, IIlya Installs Saber to fight Darius again but was once again proven ineffective until she manages to effectively strike him with Rule Breaker from the Caster Card. Illya continues to use the Saber card when fighting the Mud Heroic Spirits produced by Pandora's Box.
When Illya and her friends confront Julian Ainsworth and Erika Ainsworth , Illya battles Julian by Installing herself with the Saber Card. When Illya begins to corner Julian on how he's lying to himself of his true wish, she dons a cloak that Saber wears as a king while declaring she won't lose to a liar.
Artoria is said to be called most excellent Heroic Spirit through her attack power, defensive ability, reaction speed, magical ability, and Noble Phantasms. As the blackened Heroic Spirit, she utilizes a black barrier mist made of dense magical energy as both an offensive and defensive ability. It automatically blocks all magic attacks directed towards her, and she can direct it with her sword to launch it as a projectile at the enemy. It is strong enough to easily pierce through powerful barriers, and even if it is blocked, her swordplay with Excalibur is able to easily overwhelm all opponents.
Although she possesses Invisible Air, a wind bounded field used to hide her Holy Sword Excalibur, the sword always appears in its released state because there is no reason to hid it. It normally covers the sword in many layers of wind that obscure it by changing the air's refractive index. Excalibur is at the pinnacle of Holy Swords, able to convert the owner's magical energy into light. Increasing its momentum through convergence and acceleration to the utmost limit, it releases the energy in one slash that is said to reach the level of Divine Spirits. The blackened Heroic Spirit possesses a blackened Excalibur due to it being an amplifier that converts its owner's magical energy.
Installing the Class Card grants Artoria's abilities, armor, and Noble Phantasms. Miyu Edelfelt's Install is more reminiscent of the King of Knights, while Illya's is more to the style of the Princess Knight. 
Utilizing Kaleid Install with both Kaleidosticks brings about nine copies of Excalibur that are simultaneously released.
 Fate/kaleid liner PRISMA☆ILLYA 3rei Volume 2, p.97 - Saber
Excalibur: Promised Sword of Victory: A++
The pinnacle of the "Holy Sword" category. Owner's magical energy is converted to light, increasing it's momentum via convergence and acceleration to it's utmost limits before releasing it in one slash. It is said to reach the class of Divine Spirits. Because it is an amplifier that converts the owner's magical energy, when blackened Saber used this Holy Sword, it was black as well.
Obtains King of Knights Artoria Pendragon's abilities. Britian's legendary King Arthur who pulled out the selection sword. Attack, Defense, Reactions, Magical Ability, and Noble Phantasm are all lend to her being called the most excellent Heroic Spirit. A Heroic Spirit with a serious personality as straight as an arrow, however, personality-wise there are many sides to her as well. The figure the time Ilya uses her is not in the role of a King but rather the strong expression of her Princes Knight side.
Invisible Air: Bounded Field of the Wind King: C
The wind bounded field that covers the Holy Sword in many layers. Usually, it is used to hide the excessively famous holy sword's shape by changing the air's refractive index. However this is no such need in this work so the Holy Sword is normally in it's released state.
「聖剣」というカテゴリにおける頂点。所有者の魔力を光に変換し、収束・加速させ運動量を極限まで増大して 放たれる究極の一閃は、神霊クラスにも手が届くと言われる。所有者の魔力を変換する増幅器であるため、黒化 したセイバーが振るっていた時はこの聖剣もまた黒く変色していた。
夢幻召喚 - The original manga volume version included Rider's description by mistake, and was later corrected.