Bertha Marilla Blythe, better known as Rilla, was born to Anne and Gilbert Blythe in 1899. This event features in Anne of Ingleside and can be securely dated by looking at Rilla's given age during World War I as seen in Rilla of Ingleside. Rilla plays a minor role in both Anne of Ingleside and Rainbow Valley. During her early days she is nicknamed 'Roly-Poly' because of her chubbiness and she possesses a noticeable lisp. She idolises her Sunday School teacher and would do anything to impress her. Young Rilla has quite a high opinion of herself and prefers to wear fancy clothes, if at all possible. She hates to do anything she feels makes her look common, and this leads to quite a few misunderstandings. Her vanity is well-known amongst her family and friends and leads to her being chased down the street by Mary Vance and a codfish in Rainbow Valley. This incident deeply humiliated Rilla, but was seen as a positive thing by her mother who worried that Rilla's vanity was not a good thing.
In her late teens in Rilla of Ingleside, Rilla maintains a slightly high opinion of herself and is devoted to the sunnier side of life. Unlike her siblings, she has no desire to go to College or to teach. Intead, Rilla wants to enjoy herself, plan parties and buy new clothes. Her plans are interrupted by the outbreak of World War I. As her family and friends enlist in the army or in Women's organisations, Rilla is left at home. Before leaving, Kenneth Ford, who Rilla has always liked, asks her to promise she will wait for him. Rilla, slightly stunned, agrees. As the war progresses, Rilla begins to grow up and her character matures. She becomes instrumental in organising the remaining young people in the community to good work and adopts a war baby. Despite having no feelings for the baby at first, Rilla comes to care for him deeply and is sorry to give him to his father when he returns. Life is far from easy, but Rilla makes the most of it, realising that parties and clothes are not as important as she once thought and largely curing herself of her vanity. When her brother, Walter, dies at the front, Rilla is devastated but provides comfort to both her mother and her friend, Una Meredith. When the war ends, Rilla Blythe is no longer the flighty young thing she once was. Overjoyed to have her brother and sisters back again, she chides herself for ever believing that Ken Ford liked her, having read his name in a fortnight old list of returned soldiers. However, Ken has not forgotten her and arrives at Ingleside unexpectedly. He is much impressed at the changed Rilla and once more asks her to be his. At this moment of deep emotional stress, Rilla's lisp returns.