Jagara is an ancient Thunderian sorceress and the sole keeper of the Mighty Gyroscope, the machine in the Great Beneath that holds New Thundera together. She is neither a member of the ThunderCats team nor an enemy of it. She is the reason the world the later series is set on continues to exist.
New Thundera was reformed out of a destroyed planet and it does not hold itself together. In a cavern beneath the surface, a vast rock-hewn gyroscope spins on its axis, and if it wobbles far enough into its restraint stops the planet loses its gravity. Jagara watches it from a control post carved into the cavern wall, corrects it, and has done so, by her own account, since time began. There is no team and no relief.
She is introduced across Return to Thundera Parts I to V, where Mumm-Ra works out that controlling her means controlling the planet and takes the form of Jaga to deceive her rather than face her. She appears again in Frogman, and gets two episodes that are genuinely about her: Screwloose, in which she finally admits she cannot go on alone and asks for help, and Well of Doubt, in which she travels as a Sphere of Light, shields the group from Baron Tas's Guardsmen, and draws the Sword of Omens out of Lion-O's Claw Shield to bring him back to himself.
The series never states any relationship between Jagara and Jaga. They know each other, she is drawn as his counterpart, and the canon says nothing further.
