In this task, you're given a story (which contains five sentences only). Your task is to find all the characters which are available in the given story.

Sentence1: Helen checked out a few items at the supermarket. Sentence2: The cashier tried to swipe her credit card. Sentence3: Helen didn't believe the fact that her credit card was declined. Sentence4: She tried to argue with the cashier. Sentence5: In the meanwhile, other customers were yelling at Helen.
Customers, Cashier, Helen