I did some more exploring around the mansion and I remembered a safe being in one of the rooms. When I got to the room, it said that it had a digi-pad on it. So what I'm assuming is that we have to somehow obtain a Rotom and use it to unlock the safe. There was a T.V. so I tried interacting with it multiple times to see if a Rotom would appear (none did). One of the diary entries say that the girl wanted to pet the fur of a pikachu, so maybe we need have a pikachu in our party? I'll need to test this.