Taunt

Everything begins to go downhill once you capture her.
She starts out as a rare Eevee the player gets by capturing her in a strange, out of place patch of grass.
She seems too overpowered for a level 1 Eevee; not taking damage, not gaining exp and having the ability to constantly crit.
The player would typically restart the game because they think Taunt not taking any damage is a glitch. She takes her full form after the player restarts the game. If the player doesn't restart the game, it ends up closing out itself, forcing the player to start it up again.

Side note: If the player doesn't save the game before restarting, Taunt is still present in their party. Once captured, she is here to stay, despite being no save state after capturing her. The following occurs under both circumstances:

After you start up the game, the player starts to hallucinate, and everything in the game is very discolored and glitched out.
You open your menu and go to your party, and there she is. None of your previous Pokemon are there, only her.
Closing out the party menu triggers the game to glitch out and corrupt. The player then gets "sucked" into the game, but in reality, the player passes out and their soul is now in the game.
Various events happen within a certain time period, the ending always being the same: The fate of the player in-game is always fatal, causing the player in reality to become comatose.