Caster's True Name is unknown. They are multiple individuals. They have been summoned as "women of servitude" for this summoning, but they have a different true form.
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Caster is one of four Udon Servants created by Leonardo da Vinci with the intention of keeping the story of the comic busy. Found by Jeanne and Nitocris while still an udon larva, they decide to feed it to recovering Gudako in lieu of having to gather ascension materials to raise it. Caster later manifests to take care of Gudako despite having been eaten, constantly giving her rice balls. This causes Jeanne and Nitocris to begin to compete with Caster for their Master's attention.
Through propagating and feeding their Master an abundance of food, their Master transforms into an engorged, giant Caster-class Servant carrying a mallet and sack.