After you team up with Klarth/Claus, you visit the Long Valley in search of Magic.
Explore the Long Valley
From Bart's house, exit to the north to enter the Long Valley. Check on the rocks in your path to break them with the Pick-axe. (If you don't have the Pick-axe, you need to go south until you are in the overworld, then go north to the village and buy the Pick-axe and the Rope from the item shop.) Keep going north, and when you see the wind blocking your path, walk to the east and talk to the Sylphs to fight them. Lightning Bolt works well against the blue ones, but you will need to use regular attacks on the pink ones.
After that fight, go south to find a chest, which contains a Blue Ribbon. Then continue north.
After the save point, go east and walk through the tornadoes. From there, go north, and use the leftmost entrance to the cave.
Go north until the wind pushes you east, toward a save point. Go east and north from the save point and get behind the hole, then check on the little wooden post to send a rope down. Climb down. (If no rope appears, you need to back the way that you came until you are in the overworld, then go north to the village and buy the Rope from the item shop.)
Follow the path, using the Pick-axe on the wall in your way. Go east past the stairs, and break the wall to find a chest that contains a Spy Lens/Magic Lens. Then go west and down the stairs. After you break down the wall, go west and go up the rope.
Before you fight the Sylphs, go south and west from the hole and exit the cave to find two chests. From right to left, they contain a Kite Shield and an Amber Cloak. Then go back in and walk up to the Sylphs to fight them. Don't use Lightning Bolt against them this time.
When they are gone, go east and north to an area that you couldn't reach before because of the wind.
You will come to an area filled with Shoki/Miasma. Try to walk around the Shoki/Miasma to avoid getting injured too much from it. Make sure your food sack is filled.
Go east from the bubbling pool and break the wall with the Pick-axe. Drag the stone and use it to cover the bubbling pool.
Now go east, past where you got the stone, and follow the path until you reach another hole where you can use your rope. Check on the post and climb down.
This next area is dangerous. If you encounter a Hell Lord, run. You can't beat it at your level. Just hold L or R so you are moving in the direction away from the Hell Lord, and keep holding it until you have run away. You might want to equip a Thief's Mantle to run away from Hell Lords more quickly. If you don't have one, there is a free one behind Bart's house. You can use Holy Bottles if you want to have fewer battles. Also, check your food sack often to make sure that there is still food in it. If you run out, you might want to exit the cave and get more food, because your health will keep going down.
Go east from the rope. From the save point, go south, then east and north, and you will find a stone. Drag it as far south as you can, then as far to the west as you can, and finally, north. Cover the rift. Unfortunately, there is still another rift.
Go east, then north, then when the path splits, go east to return to where you got the first stone. Go north up that path. Go east from the rift to find a chest that contains a Talisman. Go south and break the wall to find another rock. Drag it north and then west to cover the rift.
Now go west, south, west, then north to the save point, and go west from there to find the rope. Climb up.
Go north and open the chest to get a Rune Bottle, then go south to exit the cave.
Long Valley and Summit
Go northeast and into the next area. Follow the path to the end, and there will be a cutscene.
Klarth/Claus can summon magical spirits that he has contracted with. Each spirit does a different kind of elemental or physical damage, which different enemies are weak to. Just as with Mint, you can turn off individual summons from Klarth/Claus's skill menu.
It is recommended to set Klarth/Claus's tactic to "Don't Call Spirits" unless you are in a boss battle, to conserve his TP.
His tactics are Call Many Spirits, Call Spirits, Conserve TP, and Don't Call Spirits.
Go back the way you came. Go south from the cave entrance and you will find a chest tucked in a corner. It contains an Apple Gummy/Apple Gel/Apple Gummi. Then go west as far as you can to find a chest that you couldn't reach before because of the Sylphs' wind. It contains a Charm Bottle.
Now go south to Bart's house, and there will be a cutscene. Rest in Bart's house if you wish. Your next destination is the tree in the Spirit Forest south of Beladum.
You can stock up in Harmel and/or Euclid on the way to Beladum.
Go south through Beladum and go south from there to the Spirit Forest. Go south to the water, then east, then north, and check on the tree to talk to Martel.
Overworld, part 2
After the cutscene, go north, through Beladum, then through the mountain pass.
After you go to the town north of Bart's house, you won't be able to rest in his house anymore, so be sure to rest there if you need to.
When you are ready, go south from Bart's house to return to the overworld, then go north into the town. Go north through the town and you will find Lia. Talk to her and she will join you as a non-playable character—she will not join you in battles. Go north from Harmel to Venezzia.