Constantly debated among Sentai fans is rather the characters of Nai & Mea from Magiranger and Rio & Mele from Gekiranger are actual heroes/heroine. All four of these characters were villains for about 95% of their respective series.
Nai & Mea (a play off of the word Nightmare) were the split personality of Vancuria, the Infershia's spy. They could combine into Vancuria or split into Nai & Mea. At the very end of Magiranger, after whitnessing Dagon killing Sphinx, they were shocked and had second thoughts about the Infershia. They would later revive Hikaru (Magishine), Isamu (Wolzard, who was the Magiranger's father and originally part of the Infershia), and Smokey the Cat.
Rio and Mele were the main villains in Gekiranger. Rio had revived Mele from the dead, and Mele loved Rio every since, following him in his footsteps. Rio's parents were killed when he was a kid. Master Shafu then adopted him and raised him. Rio would turn his back on Master Shafu and thought of nothing else but to become stronger by any means, even if it meant using the evil powers of the Rinjyuken school. However, little did Rio know that this was planned by another Villain, Long, who killed Rio's family and used Rio as a puppet in hopes that Rio would become the God of Destruction. After finding out that he was used as a puppet his entire life, Rio had seemed rather withdrawn from the world. Mele wanted revenge against Long but failed and was captured. After finally being convinced by Jan (GekiRed), Rio went to rescue Mele. Later, Rio and Mele burned down the Rinjyuken school and had actually planned to die in the Confrontational Beast Hall, where they were interrupted by Long. Rio and Mele then sacrificed their lives to save the Gekirangers from destruction. Later, in a dream, Rio, Mele, and some other villains that were previously defeated showed the Gekirangers how to defeat Long. When Long was revived from his seal in Go-Onger vs Gekiranger, Rio and Mele were breafly revived to defeat Long for good.
Sentai fans who argue that Nai, Mea, Rio, and Mele were heroes argue that they were heroic for their efforts (such as Nai and Mea reviving Hikaru, Isamu, and Smokey the Cat, and Rio and Mele for sacrificing themselves and helping the Gekirangers to defeat Long). Other Sentai fans argue that they're not heroes because they were villains for their respective series in its near entirity.
Personally, I would argue that Isamu is more of a hero than any of these four, since he was originally on the side of good before N Ma turned him evil. He would become good again by the near end of Magirangers. I wouldn't really consider Nai and Mea to be heroine because all they did was revive some of the Magirangers, and didn't really fight on their side. I wouldn't exactly consider Rio and Mele to be heroes either, since they were the two main villains for almost the entire Gekiranger series, but they're more closer to being true heroes than Nai and Mea. They teamed up with the Gekirangers during the movie, and their two team-ups, and actually did fight against Long who had manipulated Rio in the first place.