Within the first year of the teleseries XENA WARRIOR PRINCESS, the producers came up with a yin to Xena's yang: Callisto, vengeance-driven warrior-maiden.
In so doing Callisto's creators followed a pattern not unlike those seen with classic comics-heroes. If Superman's power is brain, give him a mortal enemy who's all brain. If Batman's tightly wound, give him a lunatic clown as a nemesis.
In part Callisto-- referred to as "the psycho Barbie" by some pundits-- follows the Batman pattern. Because Xena's entire raison d'etre revolves around doing good in restitution for her past evil acts, Callisto makes the perfect counterpoint given that she's a victim of one of those evil acts, but one who refuses to grant Xena any forgiveness.
And in addition to being just as skilled in "Greek kung-fu" as is the heroine, she's Joker-crazy as against Xena's sober-sided toughness.
Unlike the Joker, the producers apparently wanted Callisto to complete a decisive character-arc, rather than being a routine sparring-partner. This proved a wise decision, resulting in a strong dramatic sequence-- ironically not on XENA but on the companion-show HERCULES-- in which Callisto has a time-traveling experience which roughly puts her in Xena's sandals, so to speak.
Further, Callisto finally does achieve an appropriate revenge upon her enemy, but in keeping with the philosophical sensibilities of the writers, it brings her no satisfaction. Having reached that apogee, the character only appeared a few more times in an altered form (as a kind of angelic presence) before the series concluded.