GBA compression, also I recall messing with the text one time in one of the Dragon Ball ? games on the GBA and it had some oddities to it.
Anyway basic GBA compression.
The GBA's BIOS includes routines to decompress a few different types of compression. For the most part it is the LZ compression types, maybe with an unpack if you are after a font.
Said LZ is split two ways
2) VRAM aka where the graphics go.
Some will use automated scanning tools like NLZ http://www.romhacking.net/utilities/529/
, crystaltile2's compression, unlz advance http://www.romhacking.net/utilities/362/
Others will use an emulator that takes BIOS logs. http://problemkaputt.de/gbatek.htm#biosdecompressionfunctions
Said logs include what type it is and where it is found.
Not all games will use the BIOS to handle the decompression (it is a bit slower than what you can do in normal code) so BIOS logging won't always give it away. The scanners might still find it.
At times like these I also like to link up http://members.iinet.net.au/~freeaxs/gbacomp/#Image%20Compression%20on%20GBA
Finally we have tracing https://www.romhacking.net/documents/361/
though most will use the debug version of http://problemkaputt.de/gba.htm
This will find what you want, however it is not as easy to do as the earlier things mentioned.
Once you have the location you can pull it from the ROM and decompress it with a number of tools, or indeed maybe even get a tool which decompresses directly from the ROM if you feed it an address. For compression I like the GBAcrusher program, you can find a copy on the headspin's compression page I linked just a little while ago.