On super Mario sunshine where all does imposter Mario show up?

Answer 1

"Delfino plaza, Bianco hills, Gelato beach, Ricco harbor, Pinna park, noki bay, sirena beach, and pianta village." as Shawn cy said, the 'impostor' (proper name: Shadow Mario) will appear in these places, although he/she/they forgot to mention that he/it (not sure, as it's really Bowser Jr. with a weird blue neckercheif pulled over the lower half of his face, but that means that Shadow Mario doesn't really exist. confusing, I know, because I confuse even myself sometimes, and most of those times, my mouth is on auto-pilot) only appears in the seventh mission on each stage.