Seeing that all I'm doing at the moment is playing on Castle Crashers, I've decided to post helping things for people starting with all the characters. So, there you go:

Red Knight - Starter Character.
Skeleton - Complete the game as Red Knight.
Panda - Beat the game with Skeleton.

Blue Knight - Starter Character.
Mechanic - Beat the game with Blue Knight.
Fencer - Beat the game with Mechanic.

Orange Knight - Starter Character.
Red Dragon Knight - Beat Game With Orange Knight.
Ninja - Beat the game with The Red Dragon Knight.

Green Knight - Starter Character.
Royal Guard - Complete the game as Green Knight.
Saracen - Beat the game with Royal Guard.

Barbarian - Complete the "King's Arena".
Beekeeper - Beat the game with The Barbarian.

Thief - Complete the "Thieve's Arena".
Forest Knight - Beat the game with the Thief.

Periwinkle Knight - Complete the "Volcano Arena".

Peasant - Complete the "Peasant Arena".
Civilian - Beat the game with Peasant.

Eskimo - Complete the "Icy Arena".
Brute - Beat the game with The Eskimo.

Grey Knight - Defeat the Barbarian Boss.
Black Knight - Beat the Game with Grey Knight.

Alien Hominid - Have the full game of Alien Hominid HD on your Xbox 360 HDD and get at least one achievement on it.

King - Download the "King Pack" DLC.
Open Helmet Grey Knight - Download the "King Pack" DLC.