>>56580169All the partner options do have unique dialogue for talking in dungeons, which is different from the 3 dialogue sets in the overworld. It's only 4 unique lines, however. (100%, 75%, 50% and 25% health)
A while back I used an AR code to force Eevee as the partner during the select, the dialogue set was "boku" (like the Pikachu example earlier) and talking to it in dungeons just shows a placeholder message, so there doesn't seem to be much leftover data regarding the 6 hero-only starters. However, the portraits and cutscene animations did work fine, including the stuff you'd normally never see like pic related.
I'm curious if the Japanese version has more partner leftovers for those starters that may have been discarded during localization, but I haven't been able to find codes for it.