A quick note on names, I've included the "English" equivolent of their names to the left (or right... or below... or somewhere else), and the Katakana version to the right.

SPOILERS ALERT!
The descriptions below contain story spoilers.

The Good Guys

Pandora: 
     You'll hear a lot about "Pandora's Box" in this series. In this case, Pandora is the mother of both Hamel and Sizer. Currently, she is being held prisoner by Hell King Bass in a large ice crystal.

Hamel (Hameru): 
     The cursed son of Pandora (a good woman) and an evil demon named Kestra.
     Hameru is the Violinist of Hameru. Under his hat, is a large horn that appeared in his adolescence. It's caused him a lot of grief, he and his mother had to move a lot when he was young because of the curse of his and his sister's traits. Hameru is Sizer's (Saiza) brother, she was separated from him when he was very, very young.
Every once in a while, sometimes by accident, and sometimes by the intervention of the enemy, Hameru transforms into a great deamon, slaying those around him. He has the power to move hearts and amplify the strenght inside a person's soul with his music. His favorite "spell" is Marionette Version, which can temporarily raise the dead, and amplifies Princess Flute (Furutto Hime) into a war goddess, at the expense of some of her life.
     In the manga, well, he's one greedy @#$@#*@. That's all I can say. In the TV series, well, he's pretty noble. Big difference desu ne?
     One more thing... he's not very fond of doves. (evil snicker).

The Princess Flute (Furuto):
     The daughter of Horn (Horun), Ruler of Sforzendo.
     When her back is exposed to light a red cross "burns" into the skin on her back. This is the symbol of the Sforzendo family line. She's a powerful healer, a power which she inherited from her mother. For reasons I don't know, somehow she grew up in Stacato village with Hamel (which is unlike the Manga version I'm told ( =^_^= ).
Whether that's a cross or a crucifix that stands for the family line is unknown to me, it could be either knowing Japanese manga authors.

Raiel (Raeru), the piano player in the Willy Wonka hat:
     (Okay these descrips are goofy, but what the heck) Another musical musician, and Hamel's childhood friend. He wields around a magical piano that floats in front of him. When he plays it with Hamel, they can create powerful and destructive magic. When playing solo, he calls out the spirit of water... UNDINE!
     I still can't decide who my favorite character of the series is, Hameru or Raiel. Hameru's moody... but there's something about Raieru that makes him super cool... and it's **NOT** the hat =^_^=

Trombone (Toron Bon):
     A young kid who tags along with the rest of them:
     A young adventurous boy with a lot of attacking power... probably one of the most realistic warrior-boys I've ever seen. He actually acts like a kid which is a rarity in character personification these days...
     All in all he's kind of a cool character once you get past the goofy hair and his natural bratishness. He wields a huge sword, and one his best attacks can slice a HUGE enemy in half... he leaps in the air and comes straight down, kind of like "braver" if you've played Final Fantasy 7 with one difference... Tron Bon jumps higher =^_^=

Horn (Horun):
     A powerful ruler of a magical Kingdom, who's main symbol is a cross, or crucifix (dunno). She is also Princess Furuto's mother.
     15 years ago she erected a barrier to keep the deamons away. But now that barrier has been destroyed. She's also the mother of Lute, the current body being possessed by Hell King Bass.

Clarinet (Kurarinetto):
     Queen Horn's personal body guard and one of the story's favorite bishoonen. Corinet's older brother.

Corinet (Korinetto):
     Calrinet's younger sister, and one in the manga, she likes Hamel a lot! In the anime she ends up marrying Trombone.

Percus (Pakasu):
     One of the characters in the series I really don't like. He has a monacle, some really cool powers, and happens to be one of Queen Horn's devoted protectors. He also happens to be some jerk that throws Hamel to a beast, without his Violin to protect himself.

Lute (Ryuto):
     Flute's brother, son of Quenn Horn and prince of Sforzando. Currently under the magical control of Hell King Bass. Before being possessed by Bass, Lute used to study Mazoku magic, and knows one of the deadliest spells out there, "Bloody Death Eater."