Juniper Montage is the main antagonist in the Equestria Girls half-hour specials Movie Magic and Mirror Magic.
My Little Pony Equestria Girls: Movie Magic
Juniper first appears in the second special Movie Magic meeting Twilight Sparkle and Rainbow Dash on the set of the new Daring Do movie. She introduces herself as the niece of the film's director Canter Zoom as well as his personal gofer, often tasked with getting him coffee and preparing film shoots. She is also the keeper of the keys to every area in the movie studio. Being a big fan of the Daring Do series, Juniper wanted her uncle to cast her as Daring in the movie, but he passed her over in favor of star actress Chestnut Magnifico.
When a number of mishaps occur around the studio, such as the sabotage of a volcano set and the theft of several important movie props, Juniper is revealed to be the culprit responsible. She admits that she did these out of bitterness over not being cast as Daring Do and her resentment toward Chestnut Magnifico. While she is forgiven by her uncle, Juniper gets banned from the studio. As she is escorted off the set by studio security, she looks back at Twilight and her friends with malice.
My Little Pony Equestria Girls: Mirror Magic
Sometime after the events of Movie Magic, Juniper has a job at the Canterlot Mall as a movie theater usher. Though her uncle got her the job out of pity, she is very unsatisfied with the menial position and resentful toward the Mane Seven for ruining her chances of becoming famous. While trying on sunglasses at a mall kiosk, Juniper looks in a hand mirror enchanted with Equestrian magic and sees her idealized self as a famous movie star. She becomes obsessed with this mirror to the point of vanity.
While cleaning up popcorn, Juniper discovers that the mirror can trap things inside it. When the Mane Seven appear before her, she vents her frustrations at them for ruining her life and imprisons them within the mirror. Later on, when confronted by Sunset Shimmer, Juniper imprisons her as well. With the girls trapped together in limbo, the power of their geodes activates more of the mirror's magic and transforms Juniper into a giant, monstrous version of her fantasy self.
Once transformed, Juniper becomes so lost in the fantasy that she believes the people she's terrorizing are adoring fans and paparazzi. Starlight Glimmer attempts to reason with Juniper, but she refuses to listen. When Starlight offers Juniper friendship instead of fame, Juniper undoes all the magic that the mirror created, freeing the Mane Seven from their prison and turning herself back to normal. Juniper apologizes to the girls for her actions, and they forgive and accept her as their new friend.