Tsuchimikado Kugahiko

About
The prince of the underground empire that pops up after Kunlun seems pacified. He was waiting for that moment to strike.

He seems to have an odd vendetta against Amatsugami Hidehiko and wants to take Kozuki Sayoko from him as he fell in love with her after being hit, much like Hidehiko.

His underlings seem to respect him, but little is known of his normal behaviour apart from his hate of the Amatsugami family.

He reveals that Honoka was working for him all along and that she was a great help in making Hidehiko weaken the surface dwellers. He is grateful for an easy chance at beating them, though he did not take the sun sickness into account at first.

He quickly starts building up a plan for this by shooting out dust to block out the sun's rays for a while longer. He also gets to marrying Sayoko, hoping to draw out Hidehiko to take her in front of him as he dies.

It is explained that he is the son of Hidehiko's father and an underground princess. The underground dwellers in turn are people that worked on building the island, but got buried and never had a rescue team sent. They hate the Amatsugami family for this. This is a problem for Kugahiko though, as he lives with the blood of one of these hated people in him. He feels taking down Hidehiko is the beginning of the destruction of that family and his worldly takeover.

He is ultimately beaten by Hidehiko and somewhat betrayed by Honoka as she drags Sayoko away from him due to her conflicting feelings about the two hikos. Hidehiko's party then escapes as the underground empire collapses due to them destroying the management system. Honoka and Kugahiko remain, but the somehow survive.

Their epilogue shows them coming out of the sewer months later and actually being close by the group.