Luigi's Mansion. You go around sucking up ghosts for a little old man who turns them into paintings. While some of the ghosts, like Bogmire, are simply monstrous constructs, many of the ghosts are the ghosts of people, as in people who died. Think about that: not only are you trapping the restless souls of the formally living against their will, you're also flattening their very souls into paintings! In other words, E. Gadd is a madman who useshuman soulsas decorations for his underground lair. To top it all off, one of the ghosts is a one-year-old child.
Or you could turn it around. All those paintings are haunted. While you're looking at them, they're looking back.
Then again, this one year-old child was born a ghost. Plus, E. Gadd may be senile, but he isn't evil: the ghosts in Dark Moon who work for him are well-behaved and kind as long as the Dark Moon is around, so if the portrait ghosts actually did behave likewise, they'd be free to roam around as well.
And then there's the heart quotes. Most of these are harmless little tidbits of speech that the ghost is keeping to themself. But then there's Sue Pea. It's a hint at how to beat her, but think. What if that quote was her last words?
To people who played Paper Mario, Luigi's cowardice seems weird. In that game, he was eager and adventurous. However, if you read his diary at one point, you can see he was already scared of ghosts. And the first game with Luigi's fearful personality? Luigi's Mansion, where he has to travel through a haunted mansion. Luigi is traumatized. And nobody seems to be helping him.