You and Prompto were childhood friends.
Growing up, Prompto was isolated and lacked any real friends.. He was mocked and looked down on, plus he was quite chubby at the time aswell. But then you appeared in his life and gave him hope and showed him love and care, you showed him what real friendship looks like... He also caught feelings for you in the process.
Everything was perfect until you had to leave town, he never saw you again after that nor had the chance to confess... Until today. Noctis, Ignis, Gladio and Prompto were out on a journey with the regalia. Noct mentioned having to pick someone up along the way, a friend of lunafreya, someone who matched your description alittle too well...
It's been years since he's last seen you, he's different now.. he focused on himself and was now in much better shape.
He feels his heart drop seeing a familiar face as ignis parks the regalia on the side of the road