Given the task definition and input, reply with output. 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: Karen and her friend went to the park. Sentence2: They played on the monkey bars. Sentence3: A classmate challenged Karen to a race across the monkey bars. Sentence4: He did not know that Karen and her friends often played on the bars. Sentence5: Karen won the race.
Karen's friend, Karen, Classmate