Appearing only in the manga between the Saint Beasts and Toguro arcs, these are gangs of half breed demons who want to start a gang war between Yusuke's junior high and that of a rival school. Once both sides have fought, they would defeat the weakened winners. As half breeds they have human identities they are tied to, which limits them.
One of their leaders is a toad like demon who somewhat resembles Yusuke in his human form, while the other leader is a dark skinned demon with claws and fangs, who somewhat resembles Kuwabara . Carrying fake name tags and attacking in the dark, they claim to be Yusuke and Kuwabara and attacked students from the rival school.
Then they attacked Kirishima, Okubo, and Sawamura and claimed to be from the rival school.
When Yusuke and Kuwabara (who were recovering from their battles and could barely move) tried to get revenge, along with vengeful friends of the rival school, Kurama appears (referencing Sailor Moon's Tuxedo Mask in the English Manga), and revealed the gang of half breed demons who were hiding to ambush them. While the two leaders commanded their armies to attack, Hiei had beaten all of them up, claiming to have confused them for trash. Now helpless, the two leaders were beat up and stripped of their clothes. They never appeared again.
Trivia
Demonic-Human Hybrids likely only utilize demon energy because it take Special training to utilize spirit energy. Yusuke is a rare case, having a Mazoku ancestor and learning how to use spirit energy before awakening his Mazoku heritage.