I'm pretty sure you can go around the right side of the hotel.
If you mean the girl dressed in black, go to Route 210 and give the Psyduck the SecretPotion.
Otherwise, I don't know.
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you go to the team galactic person and chasse him
Use a Pokemon to Fly to Acuity Lakefront from Celestic Town. If you don't want to Fly whatsoever, then go west into the tallest mountain, and then going north all the way, then past the whole snow and blizzard.
you can find it if you have the item finder look by the door where you first come in by the patoria city After you see the girl missing the key go down until the only way to exit is through that little hotel check in place dont go in. in front of the door use the item finder and its there.
Defeat Commander Saturn at Lake Valor , Fly to Twinleaf town and defeat Commander Mars at Lake Verity and then once you return to Mt.Cornet on your way to Snowpoint city the Galaticmen will not be there.
If you caught Dialga pick up the pokeball at the back the insideit there will an orb that will raise the power of Dialgas water and steel type moves if he's holding it if not just exit the spear pillar pillar, make your way down to the snowy place then fly down to veilstone city if you can't you'll have to make all the way down to the bottom then walk to the place in valor lakefront that you preiviosly coudln't then make your way to Sunyshore to beat the last gym.