Ryoga is something of the recurring antagonist of the story. He is also Joshua Jusenkyou's father.
Ryoga's power and long life (he is known to have existed since the Age of Darkness) make him somewhat of an enigma. Throughout much of the series, he is called "The Prince of Darkness", and known to possess Underworld magicks and a secret fortress on the river Styx (to which he briefly took Joshua). This has led to the conclusion that Ryoga is some sort of demon or underworld deity; he is also known to call upon the Dark Lords.
In the short story Aspect of Death, it is revealed that Ryoga is one of the last two surviving Iname, and originally a 'receptionist' by trade. It was he who slew Draco in the Sixth Age; implications exist as well that he killed Natalie Iaggi, sister of Nicole Ransom, in the Fourth Age.