Saber's True Name is unknown. Saber is the unborn child rather than the mother.
Saber is an unborn child within their mother.
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Saber is one of four Udon Servants created by Leonardo da Vinci with the intention of keeping the story of the comic busy. Found by Artoria while still an udon larva, she infuses it with her spirit to insure it will become a Saber-class Servant. She gives it to Mash because it will "surely bring harmony to the world" if she raises it. After believing herself to having not raised Rider correctly, Mash is hesitant, but decides to take Saber after noticing Artoria slowly eating the udon.
After Saber manifests, she learns from Artoria that the true Servant is the child rather than the mother. Jeanne d'Arc arrives shortly after, believing the child is the coming of her lord and rejoicing. In later strips this is proven to be untrue however, as the child speaks from inside the womb to propose to the three Servant girls, much to Jeanne's shock.