Taeyeon is the first to realise: 

Tiffany and Taeyeon met in their 5th year. Tiffany likes the Courtyard, decorating her dorm, and meeting new people. Taeyeon likes the library, visiting the owlery, and training with her teammates. They couldn’t be more different, and so Taeyeon hadn’t been particularly keen when she was first informed that the empty bunk in her room would be occupied by the late transfer. At that point, they had only been briefly introduced as new students to house prefects. Taeyeon had recalled sitting through a strenuous sorting, only exacerbated by Tiffany’s shaky accent. By the bubbly, lively conversation they had engaged in, it had come as a shock for Taeyeon to see the house placed her in Slytherin. 

But within a month of knowing the girl, Taeyeon learnt exactly how deceiving looks could be. There must be something to Tiffany because, despite Taeyeon’s uncertain approach to her, she had found herself cooped up waiting for time to pass less and less and going outside and exploring parts of the Hogwarts grounds she hadn’t even known existed. She spent less time staring at her ceiling, and more time lying down in the grass underneath the large Juniper tree. She said ‘yes’ more often to the girls who asked if she wanted to hang and she learnt how to use a camera and took up photography. The cold mornings were less frigid and unwelcoming, and breaks signified a smile to look