@ zuccarello. I'm the person my boss goes to to cover people's shifts. Mainly because of how my hours are and the fact she can trust me to show up. But I'm not the only person that hates how this girl just leaves early. I'm sure my boss has mentioned it to her but nothing changed. I can understand wanting to leave early if it's slow but not when it's just calming down from being crazy and there is cleaning to do.
definitely not a bad person. I'm the same way as you where I always feel obligated to pick up everyone else's slack. It's so frustrating. And you're right, this behavior would not fly in another job.