Pandora
Pandora is hard of hearing, and wears hearing aids. She tries not to take hearing breaks because her classmates and teachers get irritated. Because of this, the only times she takes hearing breaks is when she is alone.
Regulus, her best friend, asks her why she doesn't take hearing breaks during the day, and when she explains, he talks to Dorcas, Barty and Evan, and the group decide to learn sign language for her, as a surprise. It takes about a month, and Barty finds it more difficult than the others, needing to keep a slower pace, but they eventually are confident enough to show her. During the classes they share with her, when she feels the need to take a break, her friends will translate for her, and when they are alone, sometimes they will spend hours only communicating in sign language.
They also use it when they want to talk about something private in public. Regulus especially likes using it as a secret language, because he feels more secure having a secret form of communication. (This is another reason why he likes learning languages that his friends know). He and Pandora both use it as communication when they go nonverbal as well.