Changing the Starter Pokémon within Pokémon Red (pokered)

Taking a closer look through the pokered source, I stumbled upon the Starter Pokémon choices and thought how it would be possible to change these. In this post I will document how I went about customising the Starter Pokémon choices available in Pokémon Red, and the modifications required to the ‘pokered’ source.

I started off by updating the Starter Pokémon constants that are present in constants/starter_mons.asm.

- STARTER1 EQU CHARMANDER
- STARTER2 EQU SQUIRTLE
- STARTER3 EQU BULBASAUR
+ STARTER1 EQU MEW
+ STARTER2 EQU PIKACHU
+ STARTER3 EQU SNORLAX

From here, I then generalised the wording Professor Oak uses when you are selecting the Starter Pokémon. Instead of the specific ‘So! You want the {type} Pokémon, {name}’, I reworded it to ‘So! You want this Pokémon?’. As this required multiple disparate changes you can see the modifications in their entirety within this commit.

Next up was to update the Rival’s Pokémon party, based on each location we encounter them. This is defined within data/trainer_parties.asm under the sections Green1Data, Green2Data and Green3Data (an homage to Pokémon Green).

Green1Data:
- db 5,SQUIRTLE,0
- db 5,BULBASAUR,0
- db 5,CHARMANDER,0
+ db 5,STARTER2,0
+ db 5,STARTER3,0
+ db 5,STARTER1,0
; Route 22
- db $FF,9,PIDGEY,8,SQUIRTLE,0
- db $FF,9,PIDGEY,8,BULBASAUR,0
- db $FF,9,PIDGEY,8,CHARMANDER,0
+ db $FF,9,PIDGEY,8,STARTER2,0
+ db $FF,9,PIDGEY,8,STARTER3,0
+ db $FF,9,PIDGEY,8,STARTER1,0
; Cerulean City
- db $FF,18,PIDGEOTTO,15,ABRA,15,RATTATA,17,SQUIRTLE,0
- db $FF,18,PIDGEOTTO,15,ABRA,15,RATTATA,17,BULBASAUR,0
- db $FF,18,PIDGEOTTO,15,ABRA,15,RATTATA,17,CHARMANDER,0
+ db $FF,18,PIDGEOTTO,15,ABRA,15,RATTATA,17,STARTER2,0
+ db $FF,18,PIDGEOTTO,15,ABRA,15,RATTATA,17,STARTER3,0
+ db $FF,18,PIDGEOTTO,15,ABRA,15,RATTATA,17,STARTER1,0

Above you can see the changes I made to the Green1Data section. I decided to keep the change-set simple and not evolve any of the Rival’s Starter Pokémon (i.e. Pikachu) throughout game-play. If you wish to do this however, it is within this file that you can decide how they evolve (or maybe even change!?).

The final change that I made was to the Rival’s Starter Pokémon move-set in the final battle, found in engine/battle/read_trainer_party.asm. In this battle each Starter Pokémon has a specific move made available to them. Again, for the sake of simplicity I normalised this to only include Hyper Beam regardless of choice.

; starter
- ld a, [wRivalStarter]
- cp STARTER3
- ld b, MEGA_DRAIN
- jr z, .GiveStarterMove
- cp STARTER1
- ld b, FIRE_BLAST
- jr z, .GiveStarterMove
- ld b, BLIZZARD ; must be squirtle
- .GiveStarterMove
- ld a, b
+ ld a, HYPER_BEAM
  ld [wEnemyMon6Moves + 2], a

With these changes applied and the new ROM compiled, I was now able to see the results of the updates above.