In the novel Five Little Pigs, Cecilia Williams ("had none") was the devoted governess of Angela Warren. She was described as a "nice, capable woman". She lived in No. 584 Gillespie Buildings, a small, cubic flat. She worked at Alderbury, the home of Angela, her sister Caroline and Amyas, Caroline's husband.
Despite having an artistic training herself, Cecilia is not very fond of artists.
Role in the novel (warning; may contain spoilers)
When Poirot reopens the case of Amyas' murder (in which his Caroline was sentenced to life in prison), Cecilia believed that reopening the case wasn't necessary; she believed that Caroline had killed Amyas.
Cecilia also told Poirot that she had seen Caroline trying to clean the poisoned bottle of beer that was used for Amyas' crime. However, Poirot deduced that Caroline was innocent since the poison was contained in the glass of beer, not in the bottle itself. This meant that Caroline was trying to cover someone who she thought was responsible for the murder.