Sonia was exploring the fourth island when Monokuma orders everyone to gather at the train leading to the Fun House. Since she was close to it, she goes there and arrives first. So she decides to sit on a bench and drifts off in her thoughts, tho she's unable to think about anything else but you. She thinks about how the soulmate mark on the back of your hand is exactly identical to hers, meaning that you two are soulmates.