A small ball like object found in the middle of Yukkuri; acts like the 'brain' or 'heart'. Often depicted as being perfectly round.
Damaging or removing it tends to result in the Yukkuri's death, but situations that leave the Yukkuri in a retard-like state have been shown. Conversely it's often shown that a yukkuri can survive as much abuse as possible as long as their paste core is unharmed.
However, many artists simply draw their yukkuri without paste cores and just filling.