Though a high-ranking officer in Mumm-Ra's army, Panthera was not completely loyal to the undead galactic warlord because of his ruthless ambition and callous disregard of life. She spoke again and again to Mumm-Ra's second, Leo, that they should rebel and stop Mumm-Ra's tyranny but was frustrated when he refused and insisted that their master was spreading order throughout the galaxy.
Her compassion was made evident when Mumm-Ra intended to destroy the Star of Plun-Dar, which supported ten planets, three of which supported life. She tried to protest but was struck with energy by the tyrant and demeaned, only for Leo to defend her to spare her more pain, and she was helpless to witness the destruction of the star and its planets.
This caused Panthera to lose all faith in Leo but it was restored when he showed her the remaining scraps of the ore used to forge the Sword of Plun-Dar, with which they would forge their own sword and revolt. All these actions restored her faith in Leo, as well as began a romantic relationship between them. She helped Leo speak with the leaders of the Jackals and Lizards, and together they successfully rebelled and banished Mumm-Ra.
Not much is known about Panthera's past, only for the fact that she was one of the original ThunderCats of the past, who assisted Leo in overthrowing Mumm-Ra. It is unknown how Mumm-Ra was overthrown exactly, but it is seen when Lion-O is reliving the moments of the past while under the form of his ancestor Leo. While Lion-O was inside the Book of Omens, he would experience what Leo did in order to understand how to defeat Mumm-Ra (It should be noted that Lion-O was only reliving what happened in the book, it might not have been exactly what happened but instead a representation of what happened).
While Lion-O was under the guise of his ancestor Leo, he learns that the ThunderCats once served Mumm-Ra and then walks into a vast holding area filled enslaved animals. Lion-O/Leo hears a woman's voice and turns to see Panthera as she enters the prison area. She goes over the strategy to invade the planet where the War Stone is located and that they will use every enslaved animal, (Lizards, Jackals, Monkeys, Vultures and Tigersharks), to obtain it for Mumm-Ra. As Leo/Lion-O and Panthera begin to move the prisoners out of their cells, a riot breaks out, forcing them to fight to maintain order. Two of the leaders of the riot are taken away by Panthera to be placed in solitary confinement. After she and Lion-O/Leo place the Lizard leader Rezard and jackal leader Shen in a holding room, Panthera unlocks their shackles. Lion-O is faced with the reality that Leo was part of a rebellion fighting to defeat Mumm-Ra before he could retrieve the War Stone. Discussing their options and involvement, Panthera tells the group that Leo needs the help of the other animals in order to defeat Mumm-Ra. Later, Panthera takes Lion-O/Leo into the bowels of the ship. There several blacksmiths give Lion-O/Leo the Sword of Omens and the Thundercats Gauntlet. He then fuses the Sword with the war stone, now the Eye of Thundera, after meeting and obtaining it from Tygus.
After Mumm-Ra is finally beaten, alarms sound and the spaceship begins to fracture and fall towards Third Earth. Panthera uses the bridge's terminal and sees a magnetic disturbance between the planet and its moon that is causing the ship to fall apart, plunging them towards the surface. In the chaos, Mumm-Ra opens his sarcophagus and seals himself in. Panthera then smashes the controls so no one else can open the evil lord's tomb. The entire crew than braces for impact as Panthera gives Leo a parting kiss.
Further flashbacks in Birth of the Blades show that it was Panthera who convinced Leo to rebel against Mumm-Ra.
- There is a slim possibility that she is an ancestor of Panthro.
- Dan Norton said in one of his comments that Panthera was Leo's girlfriend, not his wife, possibly hinting that the couple wouldn't actually get married.
- In scientific classifications, panthera is the genus classification for five species of big cats: lions, tigers, leopards, snow leopards and jaguars.
- Panthera was intentionally made taller than Leo to show that he wasn't intimidated by strong women.