6. Soundtrack

5. Warrior by Demi Lovato
Ky began to lose hope of ever finding his way to Cassia near the middle of the story when many obstacles came his way while on the mission of the Rising because they sent Cassia elsewhere on her mission trip.  But he learned to keep hope, like I think Demi did when she wrote this song about her struggles.
4. Almost is Never Enough by Ariana Grande
Xander loved Cassia, and always loved her as his Match but Cassia fell out of love with him when Ky came along on her microcard after the Match Banquet, and she fell in love with Ky soon after.  So they're nearly in love, but it's only 'almost' because Cassia doesn't love him as much as he loves her.  And their relationship never works out because Almost is Never Enough.
3. Daydreamin' by Ariana Grande
When all of the people effected by the plague that went around Oria and Central, they were forced into small containment cells that were completely see-through of all four walls around, so I imagine that they were daydreaming of being in a better and happier place.
2. ET by Katy Perry
When Ky and Cassia were placed into cities nearly as far from each other as they could possibly be to finish their mission work, I'm sure they felt like they were from 'a whole other world, different dimension'.
1. Problem by Ariana Grande
Plain and simple, the gang has gone through many struggles throughout the story so far, and once they released that stress after completing some of their missions and taking down most of what was wrong with the Society they probably felt as if they had, "one less problem without them".