Disgaea 2

He is.

 Spoiler

At the end of the game, when you fight those 2 guardians or whatever, it's hinting that they're full-blooded demons. 2ndly, the scenes involving the village and what's-his-name constantly talking about Adell's parents in that chapter completely hinted that the demons they just fought was Adell's parents. Therefore, Adell was a full-blooded demon. That's why when they fought a lot, his enemies claimed he was strong, and if he was sure that he wasn't a demon.

As for Laharl, Laharl was only half-demon, Adell is full-blooded. Though I admit, I like Laharl's personality and skills better, though Laharl is an immature plantain.