Only available after restoring Reborn City
Start by going to the 2nd Floor of the Onyx Trainer School, there will be a student looking at the wall, close to the stairs, you will need to talk to him from the side, he will then get scared and run off opening the double doors on his way.
Then go to Florinias Gym, there will be a girl standing in front of it talk to her, she will then tell you about her crush and will tell you to deliver a message to her and give you a Sweet Heart to deliver as well.
Proceed to the now opened 3rd Floor of the Onyx Trainer School, there will be a Field Read-Out here on the Desert Field, Talk to the girl on the far left she will then basically refuse her and tell you to keep the Sweet Heart.
Go back to the girl in front of the gym and she'll be devastated, after talking for a while she'll give you the Incinium-Z.