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: Don wanted to improve his sales numbers at work. Sentence2: Don began doing role play with a co-worker in their off-hours. Sentence3: Don also watched inspiring and helpful instructional videos. Sentence4: Don began feeling more confident while on the phone, selling. Sentence5: At the end of the quarter, Don was the top salesman in the office!
Co-worker, Don