Käslin made her senior debut at the 2013 European Championships in Moscow, Russia, qualifying 18th into the all-around with a 51.499 and improving her final score to 53.632 to finish thirteenth overall. In early fall, she was named to the Swiss team for the 2013 World Championships in Antwerp. In qualifications, Käslin competed in the third subdivision and qualified to the all-around in twenty-fourth place, where she finished twenty-second with an overall score of 51.566.