The Yayoi family is the third family of the Duodecim. They place a massive importance on their lineage and the preservation of their family's bloodline. Their estate is located in the 4th Hierarchical City of Naobi. 
Tsubaki Yayoi is the heiress to this family, and her father is the head of it.
In the Dark War, Ayame Yayoi fought on the front lines as one of the first wielders of the Sealed Weapon Izayoi, but could not withstand the weapon and died. Her family would become one of the founding members of the Novus Orbis Librarium soon afterwards, guarding the weapon for decades. It lay deep within the Yayoi manor, and its location could only be accessed by someone with Yayoi blood.
The Yayois are one of the oldest families in the Duodecim and have the longest known history. They kept the rest of the Duodecim together and ensured the NOL's government functioned. They took themselves to be an exemplary family and believed that the best way to create new soldiers for the NOL was to have their children "breed" with only the strongest of Ars Magus users. Since the pool of Ars Magus users was small at best, it eventually led to inbreeding. This resulted in the glory of being the strongest house both politically and physically rather short-lived. Although at first they did create exceptional soldiers with spectacular talents in Ars Magus, only a few generations later, their incestuous behavior made each generation weaker and weaker until it became their weakness rather than their strength. In Tsubaki's generation, she was the only Yayoi to survive past birth, becoming the family's heiress by default. 
Because of their attitude towards their bloodline, the other Duodecim began to view them as extremists and even the NOL personally asked them to stop inbreeding. However, Tsubaki's father refused to let the tradition die. The Yayoi's only good relationship is with the Kisaragi family.  Tsubaki's father is noted to have become a changed man after serving in the Ikaruga Civil War, but his stance remains the same.  During the civil war, Tsubaki's father served as an NOL Imperial Guard. 
Like the other Duodecim, they agreed to assist Kagura Mutsuki in his rebellion against Izanami to seat Homura Amanohokosaka on the NOL's throne. Within the Embryo, the Yayoi's household was used to host Homura and the other Duodecim as a shelter.
BlazBlue: Central Fiction Library Entry
One of the families of the Duodecim. 'Yayoi' means ‘the third moon.'
Unlike the Kisaragi and other Duodecim, bloodline is the most important quality in this family. This strengthens their bonds, stabilizing their power. However, their stressing of pure bloodline also causes various problems, and their numbers are decreasing with every generation.
The current head of the family is Tsubaki's father. 
- BlazBlue: Central Fiction, Story Mode, Episode 010, Chapter 080, Sub Scenario 1
- BlazBlue: Continuum Shift, Story Mode, Tsubaki Yayoi - Regret Sin
- BlazBlue: Continuum Shift, Teach Me, Miss Litchi, Episode 4 - A family's reputation can be tough meow
- BlazBlue: Chrono Phantasma, Arcade Mode, Kagura Mutsuki - Revolution
- BlazBlue: Spiral Shift - Hero of the Frozen Blade, Chapter 6 - Hero/True Face
- BlazBlue: Central Fiction, Library Mode, Organization #03