Unhidden Power
This routine makes the game calculate and display the actual type of Hidden Power in battle and in status screens. If, for example, a Pokémon's Hidden Power type is Water, it will display as a Water-type move, just like Surf or any other Water-type move.
First, open config.s in a text editor to customize it.
Tweak the definition of free_space to be where you want the code inserted.
This is optional and entirely aesthetic, but you can choose to have Hidden Power updated to be a ???-type TM instead of a Normal-type TM by changing change_hp_static_type from false to true. There's no associated Pokémon, so the TM Case will retain whatever Hidden Power's "static type" happens to be.
You'll need to have armips.
Place your ROM in this folder and name it rom.gba.
Once you're ready, assemble with armips unhidden-power.asm; it'll generate test.gba, which is your output (rom.gba will not be modified).