The paste core is both the “brain” and the “nucleus” of a yukkuri. It is a small compact ball inside the body, usually near the crown. This organ controls both the behavior and the shape of a Yukkuri.
Authors commonly depict the paste core as a compact ball, but the color varies. Some artists depict it having the same color as the yukkuri’s filling, while others depict it with a pearl-like color. It is made of an external semipermeable membrane which contains an amount of filling with genetic and cognitive functions.
The most common location of the paste core is at the upper back section of the head, a.k.a. the crown, alternative locations include the upper middle section of the head and the center of the body.
The paste core acts as the yukkuri’s brain, controlling its behavior and secreting some substances into the yukkuri's body.
Yukkuris have a set of common behaviors hardcoded in the paste core, which become active during fetal development, which are related to their survival: eating, motion, peer identification, basic yukkuri phrases, aversion to temperature changes and water, sex drive, paternal instinct and so on. Artists usually add more basic behaviors if necessary for the plot. Of course, stupidity and selfishness cover a big part of the paste core. Some behaviors change with age, such as a koyukkuri's paternal attachment and child-like behavior.
Each yukkuri type has a set of behaviors which make them unique and influence their way of speaking and actions and define the stereotype of each yukkuri, such as Reimu's greediness, Marisa's evilness and Alice's exaggerated lust, just to mention a few.
When a yukkuri is in a situation of fight or flight, the core secretes a substance which strengthens the taste of the filling, making it sweeter, in the case of yukkuris filled with desserts, or spicier, in the case of Meiling. A hypothesis about the purpose of this taste change is to give more energy to the threatened yukkuri so it can fight or run or as a defense mechanism by making it too sweet or too spicy for most predators' tastes. If a yukkuri becomes too stressed, the core releases all of the taste-altering substance at once, the filling tastes terrible, and the core becomes deflated. This is somewhat comparable to the human hormonal counterpart, which secretes adrenaline in fight or flight situations, while an excess of said hormone can be fatal.
The whole paste core has the information of body and mind features of a yukkuri. It is possible to convert a yukkuri body of a certain type to another by simply replacing the core. The most interesting aspect of this structure is hybridization, which is performed by tainting the contents of the core with the filling of another yukkuri, resulting in a yukkuri with combined body and mind features.
Effects of damageEdit
The paste core is a very delicate structure surrounded by filling, which prevents most of the damage of blunt hits. However, it is not resistant to piercing or shearing damage, resulting either in death, if the core is completely broken, or brain function alterations, if the external membrane is partially damaged and a foreign body touches the contents. Theoretically, if the location of brain functions were known, it would be possible to stimulate them at will with electrical shocks or foreign substances.
If the core is extracted undamaged, it is possible to implant it into another yukkuri to alter it. The effects on the body depend on both age and yukkuri type:
- In yukkuris of the same type, the transplant works as a mind transfer.
- In yukkuris of different type, the transplant works as shape shift and mind transfer.
In both cases, body size is not altered, so it is possible to have koyukkuri-sized adult yukkuris and vice versa. A limitation of transplants is that recipient's body must be big enough to contain the core and some filling to cushion it and feed it, so it is impossible to transplant an adult's core into a yukkuri bud.