User blog comment:Jopjopjop/Volume 19 Spoilers/@comment-16281343-20150310114346/@comment-73.178.133.185-20150311004325

Black Warrior is the literal translation of Xuanwu/Genbu, which is usually depicted as a black tortoise with twin snake tails. It is one of the Chinese constellations known as the Four Symbols which were the basis for the folkloric Sacred Beasts. In universe, the Five Great Head Families and their relationships to the Sacred Beasts (Chinese name/Japanese name), directions and Chinese Five Elements are as follows:

Kushihashi clan -- Qinglong/Seiryuu (Azure Dragon) of the East -- Wood

Doumon clan -- Xuanwu/Genbu (Black Warrior) of the North -- Water

Shinra clan -- Baihu/Byakko (White Tiger) of the West -- Metal

Himejima clan -- Zhuque/Suzaku (Vermillion Bird) of the South -- Fire

Nakiri clan -- Huanglong/Ouryuu (Yellow Dragon) of the Center -- Earth

The strongest of the clan inherits their clan's Sacred Beast and its (Japanese) name. In other words the strongest of the Kushihashi clan is known as Kushihashi Seiryuu, the strongest Doumon is known as Doumon Genbu, an so forth with Shinra Byakko, Himejima Suzaku and Nakiri Ouryuu. By real life tradition, Huanglong is more powerful than the others and rules over them (it was also often tied to the Emperor of China). This is translated in-universe as the Nakiri clan being more powerful than the others and serving as the mediators of the Five Great Head Clans.

Nakiri Ouryuu has made his appearance in the main series as one of the new 1st year secretaries of the Student Council, but has yet to be developed. Himejima Suzaku (a then-17 year old girl) and Kushihashi Seiryuu (a glasses-wearing boy of about the same age) have made their first appearance in chapter 8 of the Slashdog novel as discussed in the Jop's Slashdog blog, which is set 4 years prior to High School DxD. Suzaku or Shinra Byakko (from Tsubaki's) clan are the most likely to appear in the main series.