Once you get defog, you teach it to a flying type Pokemon [or any Pokemon capable of learning defog]. When you are somewhere foggy, use defog and you can see where you are going. It's very helpful.
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In Pastoria, you have to go to the Safari Zone. There you go inside, meaning into the Great Marsh. Just as you walk in, there are two people walking around. Go to the one with green hair on your right and talk to him. He has it and will give it to you.
You can't delete them because they are HM's
The hidden move defog in diamond and pearl is located in the Solace ruins. Follow the directions on the sign and you will get the HM defog.
Yes if you want it on pearl
In Pokemon Diamond/Pearl, you find a old man at route 212 with a Leafeon.
Well first you need the secretpotion from Cynthia then you need HM Defog to be able to see once you get past the Psyduck