How does Priestly present Sheila in the play?

Sheila Birling is engaged to be married to Gerald. Daughter of Arthur Birling and Sybil Birling, and sister of Eric. Priestley describes her as "a pretty girl in her early twenties, very pleased with life and rather excited," which is precisely how she comes across in the first act of the play. In the second and third acts, however, following the realization of the part she has played in Eva Smith's life, she matures and comes to realize the importance of the Inspector's message.