Player sprites begin at address E45FA4
Player palettes are at address 3366C8

Most sprites are in "4 Bit per pixel" format (or 16 colours). Some graphics are in "8 Bit per pixel" format (or 256 colours).
A 256-colour dice graphic is found at address 2E4A14. Its palette is at address 2E4814. Palettes must come first, and then graphics.

Some of the story graphics appear to be compressed. "unLZ-GBA" can decompress them. I don't know where to find palettes, and some graphics appear incomplete.
https://gbatemp.net/download/unlz-gba.9571/