Pyronica is a pink glowing being 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 only 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, Pyronica indulged in criminal activities before being recruited by Bill Cipher, along with 8 Ball, Amorphous Shape, Hectorgon, Keyhole, Kryptos, Paci-Fire, Teeth, Lava Lamp Shaped Creature, and Xanthar.
In "The Last Mabelcorn", the arrival of Pyronica and the rest of Bill's comrades into Gravity Falls' dimension is foreshadowed in Ford's flashback of a conversation with Bill. When Bill mockingly tells Ford that he has betrayed him, Pyronica's laughing silhouette is seen among others in the representation of another dimension projected in the Mindscape.
In "Weirdmageddon Part 1", during the cold open, Bill introduces Pyronica and the rest of his friends to the residents of Gravity Falls. Afterwards, on Main Street, Pyronica and the others 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. They are later subjected to Bill's fury over their inability to leave the town.
In "Weirdmageddon 3: Take Back The Falls", Pyronica 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.