Yumi Aizawa  

, 相原 由美

The girl you're referring to is Shuichi Aizawa's daughter. She makes her first appearance at the conclusion of episode eighteen, where she inquires about the meaning of "off" and causes her father to shed tears. Following the time leap, she reappears in episode 31, prompting Aizawa to ponder whether Light is Kira. He notes that she was studying and advises him not to exert himself too much.