Since the overview was on the anime, Ill do a review on the manga ^_^ Which, IMO, is much better than the anime.

Hamel is a deamon child who lives with his mother Pandora. He is beaten by the children of the town at a very young age for his horn, his deamon blood. Pandora is beaten by the townspeople for boring a son with a horn. She has one other child, Sizer, who was taken away from the mazoku at a very young age (young enough to not remember exactly what happened when she grew older). Pandoras husband, Kestra, was sealed in Pandoras Box by her, after she had realeased all the evils into the world. Therefore it was Pandora's fault mazoku roam the Earth, making everyone have a grudge towards her, no matter where she and Hamel move. Though it wasn't really her fault, Kestra persuaded her to.
At the age of 6, Hamel meets Raiel. Raiel sees the children throwing rocks at Hamel, and tells them to stop, and he protects him. Raiel puts Hamel's hat back on and smiles. Hamel is confused b/c noone was ever nice to him. Raiel says he doesn't care if Hamel is a deamon at all. So they become best friends. Raiel is amazed by the fact Hamel can play amazing music (with just a peice of grass!). Hamel says his mother taught him, and invites Raiel over.

Although quite psychotic, and mean towards Raiel in the beggining, Pandora is a caring mother. She is just fearing for her son's life, because noone has ever liked Hamel. She thinks Raiel wants to use him (which is not true), so does a few crazy things to him. Eventully she teachs him how to play the magic music on the piano though.

A few months later, Hell king 1, Bass, and the mazoku come to claim their succesor, Hamel. (Hamel is the Deamon Lord Kestra's son, making him the successor to the Mazoku throne.) Pandora protects him, thus making her be sealed in Pandora's Crystal by Bass. When Hamel see's this, he turns full deamon and kills everyone in the town, even Raiels parents (Ok..so not everyone...Raiel lives ^_^).

The manga starts 12 years after this. Hamel meets up with a girl named Flute on his travels, and she joins him, and his talking crow, Oboe. Although, through most of the early mangas,Hamel uses her as a fighting tool. Along the way, Raiel finds Hamel, with feelings of vengence towards him for killing her parents. Unsuccesful at killing Hamel, he re-becomes friend's with him. They also meet Dal Sengos young prince, Trom, who joins their party.

Meanwhile, Sizer, Hamel's sister, gets brainwashed by Bass to belive her mother disliked her. Sizer becomes evil, and carries a giant scythe with her name on it, destorying 100's of towns. She becomes Hell King 3, the Hawk King. Her only friend when she was a child, is a mazoku bird, Ocarina (daughter of Oboe). Later Ocarina turns human when Guitar (a half man half dog ...creature..thing o_O and Hell King 4, Beast King) has stabbed his sword through Sizers chest, wounding her badly. Ocarina flies off to get help.

She meets up with Hamel,Flute,Oboe,Trom, and 2 others that joined the party, Clarinet, High priest of the country Sforzando (SFZ is also the country Flute is princess of), and his little sister, sorceress in training, Coronet. She says of what is going on and they all go to save Sizer. (Which is quite intresting because Sizer was pretty evil to all of them o.O)

Pandora had told Hamel along time ago when he was a child, that he had a sister named Sizer, and that if he ever found her, he must join up with her.

When Hamel sees Sizer tied to a cross, being tortured by Guitars men, everyone begins to attack. When they finally free Sizer from the chains, Pandoras crystal, Pandoras box, and all the mazoku begin to teleport out. Sizer reachs out and grabs the key from the box, causing her to get stabbed by Pandoras crystal. Fatally wounded, she falls. Luckily, Flute discovers her healing ability, and begins to heal Sizer. She sees her past while healing her, giving her an understanding towards her.

^_^ Sizer also joins the party, along with Ocarina for alittle while. Its a great series, and thats only 1/4th of it. :D I highly recommend it, because its got comedy, drama, ^_^ among other things.