I mean where can I find the hex values for:
- the select screen character portraits (on select screen/on BATTLE PLAN)
- the palette of the characters
- the names (on the lifebar)
Are these possible to change or it requires ASM knowledge?
Adding Reptile to the roster will require ASM.
You will need to add a new custom portrait for him, as well as making the Character Select screen recognize this custom portrait and also make it select the portrait, and lastly making the selection screen recognize that you pick him from the bunch and not any other character. That's full-on ASM.
Why would you want to change the palette or names of the characters?
Reptile is already in the game, all you would need to do is simply use his assets to make him a playable character.