After you meet Lisa in her dream, she asks you to find Dr. Leo's paintbrush and then go back to her.
Get the Paintbrush
Go back to the Underground Castle. Return to the elevator, go up it, and then exit to the east. You can now go south through the path that you opened in Lisa's dream.
Go east and kill the Pineapple Plants to release an old man whose house is next to the warp tile that goes to the Grass Valley shrine.
After this, open the treasure chest to get Leo's brush.
Show the Brush to Lisa
Exit from the Underground Castle and go talk to Lisa. She will read a letter from her father that says that he won't be able to see her for a while. He reminds her to take care of Turbo the dog, Lue the dolphin, Nome the snail, and Marie the doll, all of whom were once her father's pets.
Lisa begs you to search for her father even though you are a complete stranger.
Now, go to the house on the hill and equip Leo's brush. Check on the painting and you will be drawn into the painting, titled "The World of Evil".