Puyo Pop Secrets and Unlockables (GBA)

Puyo Pop, known in Japan as Puyo Puyo, was originally released for the MSX2 in 1991. It was soon followed by a version for the Famicom Disk System called Puyo Puyo Disk Drive. A cartridge version for the Famicom would be released later in 1993. All of these versions are mostly one-player games with an Endless mode and mission modes, in which the player must eliminate all Puyos from the game field by using limited pieces.

A year after the MSX2 version, Sega released an arcade version, which heavily expanded the previous versions by including a one-player story mode and a competitive mode. Ports from the arcade version have been released for many different systems, including Super NES/Super Famicom, Sega Mega Drive/Genesis, PC-Engine, Game Gear and Game Boy. However, most of these ports were only released in Japan. The Sega Mega Drive version was re-released for the Virtual Console in Japan on December 2, 2006. (Wikipedia)

Unlockables

Hidden Characters
Play as Carbunkle - Defeat Kaakun in Trial Labyrinth, get Curry Calling Bell

Play as Satan-Sama - Defeat Satan in G. Dark Prince Castle, get Dark Prince Mask

Secrets

Sound Test
To access a hidden sound test in the Options menu, defeat Kaakun twice in Trial Labyrinth to receive CARD 18.

Select 18 in Gallery mode in the Options menu, and you'll be presented with the Sonic Team artwork for the 5 Puyo. Every time you press 'A', you will be greeted with another sound effect, including the 'gloop' sound of puyo and ojyama being dropped, which is curiously absent from the game itself.

Unlock New Areas
Signs let you gain access to new areas.

The WELCOME SIGN - lets you into Hajimari Forest; you start with it.

The MERCHANT SIGN - lets you into Goldtown; win from Seriri in Hajimari Forest.

The WARRIOR SIGN - lets you into Gachinko Dungeon; win from Nohoho in Goldtown

The CONJURER SIGN - lets you into Magical Tower; win from Draco Centaur in Gachinko Dungeon.

The MONARCH SIGN - lets you into G. Dark Prince Castle; win from Witch in Magical Tower.

The last sign is the hardest to get.

Defeat Satan-sama twice to get the DUBIOUS BOOK.

Defeat Witch in magical Tower with the BOOK to get FLAME EXPANDER.

Defeat Draco Centaur in Gachinko Dungeon with FLAME to get NOHOHO RUCKSACK.

Defeat Nohoho in Goldtown with the RUCKSACK and get SCALE PENDANT.

Defeat Seriri in Hajimari Forest with the PENDANT and she'll give you the GODDESS SIGN, which lets you into Trial Labyrinth, the 6th and final area.

Here's how an expert plays the game: