Pokémon Diamond and Pearl/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 and wild. All battles here are double battles (unless you talk to one beforehand). However, you cannot catch wild Pokemon until you have defeated its partner. Cheryl's only Pokemon is a Level 15 Chansey who knows Egg Bomb and Softboiled.

Old Chateau
The Old Chateua 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 Pokedex
Gardenia will be standing where the Cutable bushes are. She will tell you about recent activities there, but seems too scared to go inside. Check the TV at night to find a Pokemon inside. Thump the TV...and it attacks.