Music 3000
Hidden skin 1
At the main menu, press Select, Left, Right, Left, Right, Left, Right, Left, Right, Left, Right, Left, Right to unlock hidden skin 1.
Hidden skin 2
At the main menu, press Select, Left(2), Right(2), Left(2), Right(2), Left(2), Right(2) to unlock hidden skin 2.
Hidden skin 3
At the main menu, press Select, Left(3), Right(3), Left(3), Right(3) to unlock hidden skin 3.
Hidden skin 4
At the main menu, press Select, Left(4), Right(4), Left(4) to unlock hidden skin 4.
Hidden music track
At the main menu, press Select, Up, Down, Left, Right, Up, Down, Left, Right, Up, Down, Left, Right to unlock a hidden music track.
The image featured at the top of this post is ©The Legend of Heroes: Trails of Cold Steel III key art / Original