Just a reminder that providing specifics on, sharing links to, or naming websites where ROMs can be accessed is against the rules. If your post has any of this information it will be removed.
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Which game are you referring to? For Emerald, data for the trainers' parties are in src/data/trainer_parties.h. If you wish to edit other aspects of the trainer, like their name, trainer image, etc., that would be in src/data/trainers.h. These files are not .ASM, they're .H files.