just go into the marsh and talk to the one kid, he'll say he doesn't need it
firstly if you have fly it be much easier to get to pastoria city once you are in that city you go to the safari park called the great march once you go inside talk to the person on the desk and he/she would ask if you wanna enter once you do all that you go outside the other end and you see two guys the one on the right has the defog thanks for your coorperation
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in the great marsh
You can find Defog in Pastoria city. Go to the Great Marsh and near the beginning, talk to the girl and she gives you it.
use the hm-Defog (found in the great marsh-the "ace trainer" person has it)
It is in the Great Marsh, you can get in there once you defeat the town's gym leader.
In the great marsh