Long is the manipulative bastard of the group. Long plays a similar role to Captain Ryuuya of Timeranger, but his role is a bit more extensive. He doesn't appear until episode 23, but had already done his fare share of damage before he appeared in the series. Long is an immortal, indestructible dragon that can take human form. Being immortal, he found life boring so he decided that he wanted to destroy the world just for the fun of it. He was initially responsible for Maku creating RinJyuKen, the evil school of JyuKen. But this was only the beginning. It was also revealed that he destroyed Jan/GekiRed's village, as well as killing both of his parents (it was initially thought Rio had killed Jan's father Dan but Rio couldn't make sense of the wounds Dan had). The shock of this event would eventually cause Jan to lose his memory. However, his most evil deed was still underway, which is where Rio comes in.
Rio is the badass of the series. tvtropes.org describes Rio as a walking, talking, CMoA tanker, and he truly is. I remember debating on youtube who would win between Rio and Dokouku, my pick was Rio. I think Rio was intended to be romanized as Lio (close to Leo) because that's a common name given to lions, which is his beast power in this series. As a young boy, Rio was adopted and raised by Master Shafu after his parents were killed on a rainy night in a forest. This drove him to an unquenchable desire to become stronger, and led to him developing a rivalry with Dan, Jan's father. This led to Rio wanting to surpass Dan, by any means, even if it meant using RinJyuKen to do it. This is exactly what he did, and he killed Dan, or so he thought. Eventually, it was revealed that Long was the person who actually killed his parents and manipulated Rio so he could become the "God of Destruction" due to his desire to continually seek strength. Eventually, after saving Mele and finally returning her affection, Rio joined the Gekirangers in an attempt to defeat Long. After it appeared that they had defeated Long the first time, he and Mele wanted to face Ran and Retsu in the Confrontational Beast Hall in an attempt to replenish their sins. However, this was used more as an excuse so Rio and Mele could die instead. However, they were interrupted by Long, and in a heroic sacrifice, Rio and Mele sacrificed their lives to save the Gekirangers from death. In a dream, Rio, Mele, and a hand full of defeated RinJyuKen generals showed the Gekirangers how they could defeat Long.
Several years before Gekiranger took place, Mele fought for the RinJyuKen side and died during battle. She was later revived by Rio and Mele loved him ever since. She was fiercely loyal to Rio, and would fight to prove her love, even willing to die to protect him. Mele was also the first to recognize that Long had mischievous plans for Rio, and when Rio did briefly become the God of Destruction, it was Mele's voice that stopped him. After recognizing Rio had been manipulated his hole life by Long, Mele attempted to fight Long one on one, but was ultimately captured and tortured. After Rio and the Gekirangers rescued her, Mele, along with Rio, joined the Gekirangers attempting to defeat Long. As mentioned above, Mele along with Rio would sacrifice themselves to save the Gekirangers from certain death. Mele was the first, shielding the Gekirangers from Long's jaws (in his dragon form).
Besides the development and the story behind these villains, the romance between Rio and Mele is quite interesting as well, even though it's one-sided for most of the series. This romance is also a key plot point in the series.