Pokémon Diamond and Pearl/Eterna Forest

Eterna Forest
When you enter, a woman named Cheryl will ask you to accompany her through the forest because of Team Galactic. She will not only fight alongside you in every battle, but she will also completely heal you after every fight, trainer or wild Pokémon encounter. All battles here are double battles (unless you talk to one trainer). However, you cannot catch a wild Pokémon until you have defeated its partner. Cheryl's only Pokémon is a Level 15 who knows Egg Bomb, Refresh, and Softboiled.

Wild Pokémon
Note: After receiving the National Pokédex, take to the forest and level it up to make it evolve into.

Old Chateau
The Old Chateau is near the exit to Eterna Forest. To reach it, you will need to use Cut to hack through the bushes blocking the way. Inside, items including TM 90, Dread Plate, and Old Gateau are waiting to be found. There are even real ghosts inside...

After National Pokédex
Gardenia will be standing where the Cut-able bushes are. She will tell you about recent activities there, but she seems too scared to go inside. Check the TV at night (10:00 pm-4:00 am) to find a Pokémon inside. Thump the TV...and it attacks.