Broken Base: The opening is the single most contested opening theme in Sentai history. Some say it's one of the best, some say it's one of the worst.
Complete Monster: Captain Ryuya, Tatsuya Asami's Identical Grandson (give or take a few dozen "greats"), was the ultimate mastermind behind everything that occurred in the series, from the initial escape of Don Dolneiro to Gien's insanity; he was responsible for the crisis that took place in the finale (he had seen two futures — one in which 1/3 of the 21st century is destroyed and one in which the 31st century ceases to exist). To make matters worse, the only reason he was trying to manipulate everything was that he had received a vision that he would die in both futures.
Foe Yay: Naoto and Tatsuya, specially in the part of the latter. Fridge Horror
Woobie, Destroyer of Worlds: Gien. He was once just a human boy that Don Dolnero befriended while hiding from a rival family. Said family found him, tortured and killed him for fun. Dolnero brought him to a friend of his to transfer his mind to a robot body, that had side effects that were driving him insane. That's right, the ultimate Big Bad of the series is a cyborg with the damaged mind of a human boy. Quite possibly one of the most tragic Sentai villains.