Cozy Glow first appears in Marks for Effort crying outside the Cutie Mark Crusaders' clubhouse. When approached by the Crusaders, she explains to them that she is having difficulty with her lessons at the School of Friendship and fears flunking out. The Crusaders, wanting to attend the School of Friendship themselves, offer to help her with her homework assignments.
The Crusaders teach Cozy Glow how to be friendly while doing nice deeds for Sweetie Drops, Mrs. Cake, and Big McIntosh. After Cozy gets an "A" on her assignment, the Crusaders assist her further by helping her study for an upcoming friendship exam, during which she learns about the values of friendship represented by the Elements of Harmony. However, Cozy fails the exam, and her very poor test answers cause Twilight Sparkle to accuse the Crusaders of intentionally bad tutoring, believing they did so out of spite for not being allowed to attend the school themselves, and bans them as punishment.
Upon discovering this, Cozy Glow turns to the school's guidance counselor Starlight Glimmer and confesses that she failed the exam on purpose so that the Crusaders would be allowed to enroll at the School of Friendship. When the truth is revealed, Cozy apologizes to the Crusaders for her misguided intentions, and the Crusaders receive honorary School of Friendship diplomas and are appointed as friendship tutors. Cozy is last seen leaving with her new friends as they set out to tutor more students.
Cozy Glow makes a few background appearances in A Matter of Principals and one in The Hearth's Warming Club just as Twilight dismisses the students for the holiday break. She appears again in Friendship University, giving Twilight the flyers that informs her about Friendship U, and later appearing after Rarity questions how Flim and Flam could have gotten a hold of the School of Friendship's lesson plans. Cozy also appears waiting in line for Starlight's counselling in Road to Friendship.
In What Lies Beneath, Cozy stays behind to help Twilight organize the library, when she notices the Young Six having a difficult time studying for a friendship test. She manipulates them into doubting their confidence in their friendships. She offers them her study notes for help. At the end of the episode, the Young Six meet Cozy upon their journey underneath the school, where she learns about the Tree of Harmony's test on their friendship skills. As the Young Six drift off to sleep in exhaustion, Cozy looks at the vent where the Tree of Harmony is hidden ominously.
In School Raze - Part 1, Cozy's villainous nature is revealed as she manipulates the Mane Six into going to Tartarus, captures Starlight Glimmer after being put in charge of the school threatened her plans, and with the help of co-conspirator Tirek, steals the school's artifacts to harness their magic and take over Equestria. In Part 2, after Chancellor Neighsay assumes control of the school being appalled by Twilight's 'choice' of putting Cozy in charge, Cozy orchestrates a student uprising to take back her position as school headmare. She also locks the Cutie Mark Crusaders in a hall closet when they try to distract her, seeing through their attempted diversion, and she tricks the students into believing the Young Six are trying to destroy the school, planning to make as many friends as possible and steal the magic of friendship, seeing it as very powerful and something she wants.
Cozy's plans are eventually foiled by the intervention of the Tree of Harmony, causing the magic she stole to return to its rightful owners. Caught between the Young Six, the Mane Six, the school students, the princesses, and the royal guards, Cozy Glow is defeated and sentenced to imprisonment within Tartarus, where she tries to make friends with Tirek.
Role in the series
It is revealed that not only is she related to HC University, but she and her entire family descended from a pony named Vibe Idol and a race of spirits called daemons, who are in between the Alicorn Gods and mortals created by Princess Consciencea, the primordial Alicorn goddess of emotion, and that Cozy Glow herself is half-pony and half-daemon. Her daemon side represents ambition and envy, which made her smart and powerful even at birth, having the ability to use magic without a horn, while also making her uncertain of her destiny and emotionally unstable. This power was first discovered accidentally by her enviously evil big hybrid sister Backgammon, who wanted to do the same thing Twilight is doing, spreading friendship, but accuses Twilight of stealing that idea, and enrolls Cozy as an undercover student after learning of her untold power. But after being sent to Tartarus, Cozy blames Backgammon for using, tricking, and ruining her, and attempts redeems herself and becomes a student again after helping defeat and reform Backgammon.