Brotherhood: Final Fantasy XV

Prompto's backstory is revealed as he is tending to a wounded wild puppy found on the road. Initially an introverted and obese student at Noctis' school, his chance encounter and care for one of Lunafreya's puppies results in him being asked by her to be Noctis' friend. By the time the two were attending high school, Prompto had slimmed down and gained new confidence, though Noctis still recognized him from an earlier encounter and accepted him as a friend.