Sister Cecilia is a member of the Saint Elspeth Citharede, she is also known as the choirmistress.
Before Cecilia joined the Abbey, she lived at Falador. She's lived at the Abbey for quite some time now. During the quest she will tell the player about the tale of Saint Elspeth and how Elspeth's holy music killed a demon. While she is leading the choir she doesn't think their music is holy enough to hold back a demon.
After the first murder during the quest, which she personally witnesses, she thinks the demon that was killed by Saint Elspeth is back. She most likely hasn't seen a demon before. While she is thinking they are all going to die, she holds up to her faith in Saradomin.
In the various bookcases around the Abbey of St. Elspeth Citharede two books can be found that belong to Cecilia, named 'A Hymn to Saint Elspeth' and 'The Martyrdom of Saint Elspeth'. The first book contains the lyrics of the song the choir sings in the oratory. The player can pick up a copy to the right of the choir, but the item will be called Hymn book.
After the quest she will ask the player if she should leave the Abbey. She will follow the player's advice and either stay or join the travelling musicians outside the Abbey. In both cases she is happy with her choice, but is curious how life would have been if she would have made the other choice.
|† denotes that a priest or priestess is deceased, ‡ denotes that a priest or priestess is no longer a Saradominist|