Okay so
Randy is male.
It makes sense that Randy showed up during the time when
michael was left behind at the water station because that is the only time when we are unsure what has happened to him. And that he was deliberately awkward about telling
others what happened during that time.
Now speculation: Michael seems angry at Randy. (I believe it is Randy!! and that he shoots at Randy even after he sees its face.) This makes it quite possible that Randy was responsible for the broken arm. This also leads me to believe that Michael knew that he was a zombie already, because he fired without any hesitation.
When Michael is questioned about the events at the water pumping station he avoids the questions, much like he does questions about his past early on. If the previous point is true, it is likely that Michael knew Randy before this all started.
After killing
cindy early on,
Riley 'knows' that Michael has lost someone too. Yet when he seems to be opening up to
Kelly he tells her he has no-one. This could suggest he is still keeping secrets about his past even when he seems to be opening up.
Michael is one shady character.Who knows how much he could be lying about. He says his parents were killed in a car crash. Harry Potters parents were killed in a "car crash". (Not that I'm saying Randy is Voldemort or anything lol)