There's no evidence of any hidden character or event in the game that fits the description. The story likely arose from the game's spooky atmosphere, particularly the presence of ghosts and the eerie music in certain locations.
It's important to remember that rumors and fan theories can be a lot of fun, but it's always best to approach them with a critical eye.
Here's what we can confirm:
* No such character exists in the game. There is no evidence to support the existence of a "Girl Who Kills Pokemon" in Pokémon Diamond and Pearl.
* The story is likely a fan-made creation. It's possible the rumor started from a player imagining a spooky encounter or from a misinterpretation of some in-game events.
If you're looking for a real challenge in Diamond and Pearl, try focusing on the game's actual difficulty and the many tough trainers and wild Pokémon you'll encounter.