After you go north through Route 205 and rest at the house there, you go north and enter Eterna Forest.
Go north and you meet Cheryl, who will join you and will automatically heal up your pokémon after every battle. When you encounter pokémon or trainers, Cheryl will join you in battle and send out a level 15 Chansey, who can use Disarming Voice, Sweet Kiss, Tail Whip, and Life Dew.
While Cheryl is with you, if you want to catch pokémon, you will have to defeat one of the two wild pokémon in order to catch the other. In Eterna Forest, you can catch Budew, Buneary, and Wurmple. In addition, in Brilliant Diamond you can catch Beautifly, Murkrow, and Silcoon, and in Shining Pearl you can catch Cascoon, Dustox, and Misdreavus.
Trainer Battles and Items
Go west from the entrance and you will find an Antidote in the grass.
Go northeast and you will battle Lass Briana and Bug Catcher Jack. Jack has a level 9 Wurmple, a level 13 Beautifly, and a level 11 Silcoon. Briana has a level 14 Pachirisu. You get 972 Pokédollars for winning.
North of them, you find a Paralyze Heal.
Go east, then south, and you will battle Psychic Elijah and Psychic Lindsey. They each have a level 15 Abra. You get 1560 Pokédollars for winning, or if you fight them separately, you get 780 from each.
Go south from the Psychics. To the east, you can battle Bug Catcher Donald and Bug Catcher Phillip. Donald has a level 9 Wurmple, a level 13 Dustox, and a level 11 Cascoon. Phillip has a level 12 Burmy and a level 12 Kricketune. You get 948 Pokédollars for winning.
To the east of the Bug Catchers, there is a Potion.
To the north, there is a sign, and north of the sign, you can battle Psychic Rachael and Psychic Kody. Kody has a level 15 Meditite and Rachael has a level 15 Psyduck. You get 1560 Pokédollars for winning.
Go northeast, then north, then west. To the north, there is a jar of Honey.
Go northeast from there and you will reach the north exit of the forest. When you go toward the exit, Cheryl will depart. Go east to leave the forest.
Route 205 North
Just as in the southern part of Route 205, in the grass you can catch Bidoof, Buizel, and Pachirisu. In this northern part of the route, you can also catch East Sea Shellos. You can use the Old Rod in the water to catch Magikarp.
Along the bridge, there are several Fishers that you can talk to, and some of them will battle you. The second Fisher from the left is Fisher Joseph, who has a level 14 Goldeen. You get 728 Pokédollars for winning.
The third Fisher from the left is Fisher Andrew, who has six level 10 Magikarps. You get 600 Pokédollars for winning.
The Fisher on the right is Fisher Zachary, who has two level 10 Magikarps and a level 13 Goldeen. You get 600 Pokédollars for winning.
To the south, there is a Potion.
To the north, there are four berry trees.
To the east of the berry trees, there is a hidden jar of Honey in the corner.
When you are ready, go east to Eterna City.