Answer:
Alliteration
Explanation:
The author employs Alliteration, or the poetic device of using 2 or more words that start with the same letter together, when describing the "mystical moist" night-air.
The first blank is protagonist, the second blank is antagonist, and finally the third blank is secondary character.
The use of the prepositional phrase is "on the roof".
On is a preposition and the phrase ends at roof.
I hope that helped!