Who is Rhiannon Calvert - Ginny's best friend, from The Broken Bridge and what is their importance?

Rhiannon is Ginny's best girlfriend. Ginny confides in Rhiannon on many occasions, to get her friend's opinion. Rhiannon introduces Ginny to the basic story behind the broken bridge, which defines Ginny's mission for herself more narrowly. Ginny's friendship with Rhiannon is one of acceptance and genuine affection.