Tatsuma Hiyuu is a mysterious boy who transfers to Magami High School. He is often shown as being very silent, amiable, and carefree, but actually he has more insight on the people around him. He usually hangs out with Kyouichi and helps him fight gangsters from other schools, giving him the status of the fifth most delinquent student in Magami High School's history. Tatsuma is probably the only one in the group who is close to Kyouichi and would often calmly tease and irritate him.

He is a skilled martial artist and fights using a "tekou" (wrist and hand armor); he knows a person's vital points, whether or not they are fighting seriously, and if they have killed before. He is often the first to realize a threat that is approaching them, and sometimes gets a strange feeling when someone near him is in danger. Unbeknownst to his friends, and adding to his mysteriousness, prior to enrolling in Magami High, it is revealed that Tatsuma was sent there by his Sensei, who said that his skills would be needed in Shinjuku.

Tatsuma carries strange birthmarks on his hands, and although it is never shown in the anime, he also carries markings on both feet as well as a larger symbol on his chest. His father told him to treasure them because they gave him powers that will one day be of important use. However, only Aoi knows of his marks. In flashbacks of his childhood, Tatsuma played games with other kids and he was instantly able to guess or find the other children with some sort of precognitive powers. This caused the other kids to shun or fear Tatsuma, making him feel different. He has had a difficult time making friends because of this.

Due to his handsome looks and habit of falling asleep in class, Tatsuma is often called "Sleeping Beauty" by his homeroom teacher. He is always seen wearing a blue hoody, with his school uniform worn over it. Tatsuma dislikes regular milk, so when he was a child, his mother mixed in some strawberries, calling it 'magic'. Since then, he is often seen drinking strawberry milk. Presently Tatsuma lives alone after traveling to Tokyo.

It is revealed that he is an orphan, and had previously lived with his foster parents. Not much is revealed of his true parents, but Tatsuma is connected to Munetaka Yagyuu through his real father ...

In the second season, it is revealed and eleborated on his connection with the Stars of Fate. He is the Vessel of the Golden Dragon, and his birthmarks connect him to the Vessels of the Four Gods who would awaken and protect the Golden Dragon.
Tatsuma is a descendant of Tatsuto Hiyuu from the prequel game Tokyo Majin Gakuen: Gehouchou. Tatsuto and Tenkai Kodzunu had battled during the Bakumatsu, and this is shown in flashbacks between Tatsuma and Tendou Kodzunu.

(Adapted from Wikipedia, Japan Wikipedia, and original video game data)