The underlying themes explore the meaning of humanity through the eyes of two major characters. As one character spirals towards losing humanity, the other evolves into it and these two paths intersect and merge on and off throughout the series.
The character we are first introduced to, Yuji Kiba, is not the title character, but whenever he is onscreen, he steals the scene. We first meet him as he's driving his parents through Tokyo, simply talking about his girlfriend. In an instant his life is changed as a truck crashes into his car. His parents die instantly, and he's thrown into a coma for a year. At the end of the year, his life support is pulled. In the morgue, however, he rises off the slab, seemingly no worse for the wear.
In this method, Yuji Kiba has become an Orphenoch, a human/animal hybrid that is meant to be the next step in evolution.
The second character is Inui Takumi, Kamen Rider Faiz. He is a jaded soul, and he at first refuses attempts at friendship. But soon, he takes on his destiny when he sees what the Orphenochs are.
In order for the Orphenochs to propagate, they have to "kill" humans. Most who are affected in this way are immediately crumbled to ash. However, those who survive, such as Kaido Naiyo, evolve into Orphenochs.
For this reason, the Faiz Rider gear found it's way to Mari Sonada, and eventually Inui Takumi. When in use, the gear can augment strength of the wearer, but only under specific circumstances. Through a laser pointer affixed to the ankle, a high tech brass knuckle and a lightsaber-esque blade, the suit can channel it's energy in what's known as "Exceed Charge." The result of Exceed Charge on an Orphenoch is a flare in the shape of the greek letter Phi, and the orphenoch crumbling to ash.
As time goes on, Takumi and Kiba's paths cross, often with the Horse-Orphenoch Kiba fighting against the Faiz-Powered Takumi either out of misunderstanding or deception by other riders or orphenochs. Both grow in power, much to the consternation of the Lucky Clover, the four most powerful orphenochs in the land.
By the end of the series, it seems that Takumi has become more human, while Kiba has become less human.
There are surprise plot twists enough to keep viewers guessing throughout the whole series. Some of these, such as "What happened at Mari's school reunion?" have repercussions throughout the remainder of the series.
I highly recommend collecting this series in proper subtitled format and not in the HK style. As with its predecessor, Kamen Rider Agito, too much is lost in translation in the HK mode.
