Harry Potter Side Quest

To start the Harry Potter Quest go to the train station in Saffron City (Northwest of the map.)
If you do not have access to the train station yet, you will need to complete two quests.

Quest 1:  Get A Ticket
There is a house in Saffron City that has a different colored rooftop.  Once you’re inside the house, The Copy Cat wants a Pokedoll.  To get the Pokedoll, you need to go to the Pokemon Fan Club in Vermillion City.  Once you bring The Copy Cat the Pokedoll, you will get the ticket to the train station.

Quest 2:  Burglar Quest At The Powerplant
You will need to go to the Power Plant and defeat the Burglars.  To get to the Power Plant, fly to Route 10, walk North until you hit the water, then Surf South.  Talk to Hank near the top middle of the Power Plant to start the quest.  Once you complete the quest, the train station will have power again.

Enter the train station in Saffron City in the Northwest part of the city. Go down the stairs and loop all the way around to get to the hidden 9-3/4 platform.

To start the quest talk to NPC Ghosty Gertrude. NPC Meg will appear, talk to her and the quest will start.

Once the quest starts you will have to gather clues to find Harry.
Clue 1: Located near the entrance to the left of NPC Maria

Clue 2: Located in the Ecruteak City Theater. (Building right above the Pokemon Center.)

Clue 3, 4, & 5: Located in the Ecruteak Graveyard.
To get to Ecruteak Graveyard, enter the Barrier Station located in the north part of Ecruteak City (3A).  Instead of entering Bell Tower, go south (3B).
Once you’re in the graveyard, go north until you see a cave (3C) and enter it.
Go down the ladder marked “Clue 3” in Clue 3D below and there’s Clue 3.  Go back up the ladder and go down the ladder marked “Clue 4” in Clue 3D below.  Clue 4 is to the left of the ladder.
AFTER getting Clue 4, go up the ladder on the left side to head to Clue 5.

After going up the ladder, follow the path go through another ladder or two until you pop out of the cave and keep heading north until you see some NPCs on the right side of you screen.  Here you will find a hidden ladder (Clue 5A).  The NPCs will get in your way a bit so be patient.

Once you go down the hidden ladder, the last Clue will be in the bottom left corner of the room.  (This room is also filled with daily NPC battles if you need to level up your trainer in the Johto region.)

Now that we have all 5 clues, it’s time to head back to the Train Station in Saffron City.

Talk to the NPC Ghosty Gertrude again to make Meg appear, then talk to NPC Meg. This will make Harry appear. Talk to Harry and Ron will appear and then a 3v3 fight will begin. You will complete the Harry Potter questline after defeating them and you now have access to the Ecruteak City Graveyard for daily NPC Battles!