An ancient race of monsters who can assume human form called the Grongi begin to murder humans in Tokyo after being awakened. The only one who can match their power is a young adventurer named Godai Yuusuke, who gains the power of Kuuga after its previous bearer passes its power to him.

From Fairgame:
Haruka is an unlucky highschooler but one day she grab the happiness. She is going to get married with her fated husband, Masayoshi. He had been married once before but he is serious and kind, and supposed to make happy family life but then...

There is a strange breed of mechanical creatures, aptly called “Machines,” who will, on the condition that their Host never love another person for as long as they live (else the debt will be paid in blood), grant their wielders fortune and prosperity. And where a Host has betrayed his Machine, you are sure to find a member of the mysterious Gari family, wielders and hunters of Machines, in the ensuing carnage…

So it was justifiable when the Gari family was dispatched to Shakunage High School to covertly investigate a brutal murder perpetrated by the spindly hands of a Machine… But it wasn’t so justifiable when Hanare Shizuma, a youth with an agenda of his own, enrolls at the High School soon thereafter, hell-bent on conducting his own investigation – and soon meets the head of the family of hunters, shockingly enough, a girl his own age. But little do either of them know that their encounter will set the cogs of fate spinning rapidly into motion…