I had to go with Ayase here, because Kozupii's cover skirts a little close.
Character songs - a way for a fictional character to sing of what is in their heart, and in a way, one of these songs gives me good reason to philosophize. Phantasm's singles, in the world of the game/show, are Ayase singing as FES. Good for her underground public image, but this Trigger is not FES or Phantasm. It is Ayase Kishimoto, the real her. The her that Takumi came across and delusioned in a swimsuit. On that note, Kozupii's is also nice for her because she communicated only by thoughts for 90% of her screentime.
Onward, and enjoy!