Why does the series have a "pumped-up, Hot-Blooded" opening theme and a "soft rock" ending theme? The final episode drives the point home—Yusuke, finally unburdened by the duty of being Kuuga, shows his kindness to a group of children with the ending theme playing over. The two musical themes of the show embody Yusuke Godai's character—a kind man forced by circumstances to become a fearsome warrior who would save the world.
The fact that three of the Grongi are based off Kamen Riders—namely Badaa, Gadoru, and Daguva—makes it practically foreshadowing for the fact that Kuuga's powers are no different from theirs.
The opening song's first few chords actually foreshadow Kuuga's forms.
The first chord is pretty normal when compared to the second through final chords, and thus embodies Mighty Form and its flexibility to most situations.
The Instrumental version replaces this low tone with two identical sounds of multiple notes in succession. The first is at a lower volume, perhaps representing Growing Form, while the second is louder (but not too loud), corresponding with Mighty Form.
The third is played in a different audio channel compared to the others, evoking Pegasus Form's Super-Senses.
The fourth is bigger and louder than the others, which matches Titan Form.
The final chord before the lyrics hit is a big, devastating riff. This embodies Ultimate Form: Kuuga at his strongest...and his most vulnerable to his destructive Superpowered Evil Side.
Usually, every main Rider series has at least two Kamen Riders, with Kuuga being the sole exception. But then you remember—Yusuke IS the second Rider in his story. Riku was the first—the original Kuuga! It's just that Kuuga is about the second Rider, learning from the first Rider to combat their shared enemy.
This is also likely the reason why Riku chose to seal himself along with Daguva. Daguva likely called out Riku on that he was no different from him and planned to make the Linto into the successor of the Grongi.
One commercial has a child being chased by a Grongi before thankfully Kuuga shows up. Anyone who watches the show will know the Grongi's intentions clearly.
Why is it only the police, and not the military, who are engaged in fighting the Grongi? Because the Grongi behave more like serial killers, not an invading horde or attacking army. There are no grand-scale attacks; simply a trail of dead bodies, by monsters who can look no different from humans. Declaring martial law in the city would make the situation worse than usual—the Self-Defense Force could feasible end up killing more civilians than the Grongi. Even if they can't kill the Grongi, the police are best suited to handle this situation by protecting the civilians.
On top of all of this, there's the fact that the Japanese government would have to acknowledge that there's ancient mutants running around killing citizens. And sure enough, the police officially confirm their existence within the first four episodes. It's a hard concept to take in, let alone say aloud for the world to hear. And if they bring in the JSDF, then there's a chance that the Gurongi could either spread out and kill elsewhere, or escalate the conflict into all out war.
Episode 20: Why does Yusuke not hesitate to burn the growing remains of #26? He's likely acting out of fear of #26, the Grongi that came very close to killing him so far. Who wouldn't do the same thing? This, and his battle against #26-B, are his only significant actions acknowledging this Grongi poisoning him; he never says a word about the Grongi itself.
Among all of Yusuke's 2000 talents, he fails with one: spooking people. As far back as Episode 1, his attempts to scare people end with the subject flatly telling him to cut it out, but it's possible he's too much of a Nice Guy to scare people on purpose like that. Eventually, however, Yusuke does prove to be scary…as Kuuga, with his growing power and growing rage... His 2001st talent.
