Brenda MacGuff is Juno's stepmother and Mac's second wife, as well as the mother of Juno's younger sister, Liberty Bell. She is portrayed by actress Allison Janney.
Bren is noted as having an extreme fondness for dogs and often collects pictures of them. Juno considers the habit to be very odd, though she later learns it is because Bren had always wanted a dog and had not gotten one because she knew Juno was allergic. Along with Mac, Bren accepts Juno's decision to give birth, though she privately expresses the same anxieties as Mac does regarding Juno's choice.
Throughout Juno's pregnancy, Bren ensures that Juno remains healthy and does what she can so that her stepdaughter is comfortable, including altering Juno's jeans so she can continue wearing them. When an ultrasound technician makes condescending remarks about Juno becoming pregnant, Bren staunchly defends Juno and criticizes the technician for having a narrow minded attitude.
As Juno begins spending time with Mark Loring, Bren warns Juno against developing a relationship with Mark as he is a married man. Though Juno disagrees and they argue, Juno eventually realizes that Bren was right when it becomes apparent that Mark and his wife, Vanessa, do not have the stable loving relationship Juno had thought they had.
When Juno goes into labour, the MacGuff family rush her to the hospital, where Bren stays with Juno in the delivery room until Juno's baby is born. In Bren's last scene in the film, she is seen in the garden with a new dog, as she has always wanted.