A Pure Heart is a type of crystal that exists within a person's heart. A person who is passionate about something or thinks about others has a Pure Heart. If a person's Pure Heart is separated from him or her for too long, then he or she will die.
Sailor Moon S
Doctor Tomoe and Kaorinite were seeking out Pure Hearts, extracting them in the hopes of finding three Pure Heart crystals: One of which would take the form of a mirror, one of which would take the form of a sword, and one of which would take the form of a crystal ball. If these three crystals were in the same place, then the Purity Chalice would appear. It was Tomoe's intent to use it to summon Pharaoh 90 so he could destroy the world. Amara and Michelle (Sailor Uranus and Sailor Neptune) were also looking for them to get the Purity Chalice to save the world.
Kaorinite sent Daimon Heart Snatchers to extract Pure Hearts from people by using star beams on them. Serena's Pure Heart glowed with a bright light and appeared to possess power, but merely reflected the purity and was not one of the three crystals.
After Kaorinite was killed, Eugeal extracted Pure Hearts using a Heart Snatching gun. She soon discovered Amara and Michelle to be two of the three holders of the Pure Hearts (the sword and the mirror, respectively) so she called them over to the Marine Cathedral and extracted Sailor Neptune's Pure Heart. Sailor Uranus extracted her own Pure Heart. Later, Sailor Pluto revealed that the crystal ball on her staff was the third (likely her Pure Heart, separated from the treasure itself). After the Purity Chalice was summoned, she separated the Pure Hearts from the treasures, returning them to Sailor Uranus and Sailor Neptune. Ultimately, Sailor Moon got the Purity Chalice, and Eugeal was killed by Mimet.
Afterward, Mistress 9 commanded the Heart Snatchers to extract Pure Hearts for her to eat, as they will give her power. Mimet sent Daimons that sucked her targets' Pure Hearts out through their mouths, but all were returned to their owners unharmed when the Daimons were destroyed.
After the death of Mimet, Telulu took over. She extracted Pure Hearts from several people at once using a plant called a Venus Heart Trap. However, on her first attempt, all her plants were destroyed by Sailor Moon except one, which turned on her and killed her.
Byruit used her nanocuff to control her master computer, connected to all the other computers, which she used to extract the Pure Hearts of the exam takers at Mugen School. However, she only got one chance at this, as Sailor Moon damaged her nanocuff, causing it to backfire and kill her, releasing the Pure Hearts and returning them.
Kaorinite, who was revived by Doctor Tomoe, brought Sailor Mini Moon to Mistress 9, who ate her Pure Heart and awakened in her true form. Darien kept Rini alive until Hotaru (whom Mistress 9 was possessing) destroyed Mistress 9 from within, and, as Sailor Saturn, returned the Pure Heart to Sailor Mini Moon.
In the final episode of Sailor Moon S, a final Daimon named Rangy extracted Pure Hearts from people for no apparent reason, seeing as how Pharaoh 90, Mistress 9, and every other member of the Bureau of Bad Behavior (excluding Tomoe, who had been possessed by Germatoid the whole time and was now free of his control) were dead. However, they were returned when Sailor Moon destroyed her.
- Pure Hearts share their names with eight items that Mario collects in Super Paper Mario (which came out years after Sailor Moon S was taken off the air). The way that Mimet's Daimons suck out Pure Hearts (through the victim's mouth) is similar to the way a Dementor would suck out a person's soul in Harry Potter. There is a villain in Super Paper Mario named Dimentio, whose name sounds similar to "Dementia" or "Demented", or, indeed, "Dementor". In addition, there are eight Pure Hearts, and Mimet sends eight Daimons (not counting Ikasaman, who had nothing to do with the Heart Snatchers' plan) to extract Pure Hearts.