It is the mastermind behind the events of the first series and the ultimate adversary who repeatedly confronts the "Three Prankster Kings." It is commonly known as "Shinigami-Gii" (a nickname coined by Ryou-chin; according to Sooryuu, it's true name is unpronounceable by humans, though this approximation comes closest).
Originally a type of angel tasked with guiding the souls of the dead to the underworld, it abandoned this duty to hunt souls and absorb them to fuel it's own power. It's might is overwhelming—far beyond the capabilities of the Three Prankster Kings, and even too great for Ruka and Sooryuu to overcome, even when they joined forces.
It is a cruel and nefarious entity who assumes various guises to possess the hearts of humans (and spirits), manipulating them for it's own ends.
