Youki

In the Hikarigakure the excess chakra will add up and begin to disturb the balance of nature. To prevent any harm the Hikarikage developed a sealing jutsu to seal the chakra inside of a person. However upon this person's death the chakra would have to be resealed, along with what ever excess chakra that had gathered after the initial chakra was sealed. Over time this amounted to be a massive amount of chakra and was deemed to massive to be sealed inside a single being. It was decided that a pair of twins should bear half of the chakra each and when it was announced that a member of the Utsukushi clan was expecting twins they decided to use the twins. The reasoning for using those twins was that the Utsukushi, long known for their massive amounts chakra and chakra control, would be able to control it without a risk of it ever being released. Upon the moment of their birth the chakra was be extracted, split into two halves, then resealed inside the twins. The oldest twin's body, not possessing enough chakra to keep the rouge chakra in check, began to reject the seal. Before the twin could suffer any more they quickly extracted the chakra and placed it inside the youngest twin's body. The twins were Kurai and Mitsumi Utsukushi with Mitsumi being the younger and Kurai being the older of the two.

Youki cannot use the chakra sealed within them. Doing so risks extracting the ckakra by accident and setting it free which would damage the area around them. Everytime a youki attempts to use the sealed chakra the seal will weaken. The only person ever capable of using the chakra without damaging the seal was Mitsumi Utsukushi. Her body suffered major injuries because in order to keep the chakra in check she had to use her Chakra bending jutsu to force her body to make her own chakra to compensate for the seal.

Youki are like jinchuriki in the fact that they contain something but unlike jinchuriki not shamed or feared for what they possess. Often time they are respected for their ability to contain the chakra.