Pyronica is a pink glowing demon with short pink hair, bangs parted across her face, and several small horns across her skull, in between her two other big curved horns. Pyronica has one eye, just above her wide mouth and thick lips, that hide her buck teeth and fangs. Pyronica has a slim humanoid body, and her limbs are made of white flames. Pyronica wears a long pink cape and pink stiletto pumps.
Eons before the events of the series Hectorgon indulged in criminal activities before being called together by Bill Cipher, along with 8 Ball, Amorphous Shape, Keyhole, Kryptos, Teeth, Paci-Fire, Hectorgon, and Xanthar.
In "The Last Mabelcorn," the arrival of Pyronica along with her demon comrades into Gravity Falls' dimension is foreshadowed in Ford's flashback of him working with Bill. When Bill mockingly tells Ford that he has betrayed him, Pyronica's laughing silhouette is seen in the representation of another dimension projected in the Mindscape.
In "Weirdmageddon Part 1," during the cold open, Bill introduces Pyronica with the rest of her demon friends to the residents of Gravity Falls. Afterwards, on Main Street, Pyronica and the other demons watch Bill turn Ford into a golden statue and taunt Dipper before burning the journals. Later in the episode, Pyronica is seen playing "Spin the Person" at the "V.I.P." party Bill is having at the Fearamid. When the calcified statue of Lazy Susan points in Hectorgon's direction Pyronica happily swallows the demon whole.
In "Weirdmageddon 2: Escape From Reality," Pyronica and the rest of Bill's friends attempt to leave Gravity Falls to spread their chaos across the world, only to collide with an invisible barrier.
In "Weirdmageddon 3: Take Back The Falls," she participates in the battle against the Shacktron, initiating the attack on her command. Following Bill's defeat, she is sucked back to the dimensional rift in the sky along with her fellow demons.