The secret characters are frodo merry pippin and faramir.
(you must have beaten the game to use codes)
to enter code pause game and hold all shoulder buttons (L and R) enter code while holding buttons. all ups/downs are on the d-pad
gimli: B B Y A
gandalf: B Y up down
merry: down down x a
frodo: down y up down
faramir: x y up x
aragorn: up x y a
sam: y a down a
pippin: y a x a
legolas: a y up a
thats +1000 exp for each character. i'll give you more later