Cesario said she and Kelly had met just two hours before the set by Jason Aldean, one of the US' largest country music stars.
"I left my friends so we could go up super close to the front for the end of the night," she wrote on a post published on the Love What Matters Facebook page.
"We were just dancing and having fun and then all of [a] sudden there were loud noises that sounded like fireworks, but no lights were going off.
"It stopped, and Jason Aldean kept playing but then the shots fired again and he ran off the stage.
"Before I knew what was going on, Brendan tackled me down to the ground and covered me from the fire. It stopped again and he looked around to see what was happening and then it just kept going."
By some accounts, the firing continued for five minutes.
"[Brendan] looked at me and said 'We have to get out of here. We can't stay here. It's not safe'," wrote Cesario.
"Then he pulled my arm up to get me out of the piles of people. We had no idea who was dead or who was alive. We just started to run. He kept telling me it was going to be okay and to keep running until we were safe.
"The whole night he didn't leave my side."
Kelly then let Cesario use his phone to call her sister, Jenn, to tell her she was safe.
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Cesario also detailed the texts her sister sent to Kelly to thank him.
"Thank you sooo [sic] much from the bottom of my heart for taking caring of my sister."
"Absolutely, that's what we do, take care of our own and those around us," Kelly replied.
"Glad I could be there for her in that crazy time."
Friends of Kelly have chimed in to sing his praises.
"Brendan Kelly is my boy, drinking buddy, and a fellow Marine ladies. I can 100 per cent confirm that he is single and not only a genuine hero, but a gentleman as well," said Chance Wilk on Facebook.
Not unsurprisingly, many have concluded the drama should lead to bigger and better things for the pair.
"OK not to be an old lady here but ... marry him!", one woman wrote on social media.