Who is Bethan from The Light Fantastic and what is their importance?

Bethan is the young woman the Druids want to sacrifice at their ceremony. When Rincewind tries to save her, she is not best pleased, saying she was looking forward to entering the afterworld. However, she becomes an important character, and she is the one who realizes Rincewind pronounces one of the spells incorrectly. Bethan falls in love with the much older Cohen, and they decide to get married.