I noticed something about Kamen Rider the other day...
 

Its the only one of the big three Tokusatsu that regularly have humans as their primary villains rather then non humans. While they obviously still fight monsters and other none humans, humans are more often then not extremely important, if not the primary, villains. Kamen Rider in general seems a lot less afraid in general to show the dark side of humanity. Its kind of interesting how Kamen Rider has retained its darker elements long after sentai has abandoned them. That later time slot makes all the difference man...

Kamen rider's powers usually originate from the villains one way or another, so riders are seen as monsters themselves in their respective series. So it wouldn't hurt to have them battle other human transforming monsters. Modern rider series tend to depict the human guest of the week being the monster of the week either through transforming from humans to monsters or vice-versa. Sentai only retain using the enforcer and final boss having a human appearance after 40 years. Overall, sentai usually deals with alien invasions while riders deal with more one on one human's problems.

Which makes sense on a scale level as well. Kamen Rider is generally a more personal and individualistic series, focusing on how individuals deal with their situation, while Sentai is group focused on the whole, seeing the clash between different factions.

...So Sentai is very Japanese, while ironically Kamen Rider comes off as a very non-oriental style hero. I mean, this is the part of the world that came up with Tall Poppy syndrome...

So... would you that Kamen Riders are, at their core, rebels?... I certainly would. I also really like how a Kamen Rider's powers is always connected to their villains in someway.

It's definitely one of the themes that, well, at least in Showa is pretty consistent. Riders are, according to the rules of their world, supposed to be the bad guys. Monsters, mutants, cyborgs, etc.

It's... actually one of the reasons I like Rider conceptually so much, due to... certain aspects of my existence, let's say. When everything about their situation says they should be a villain, they choose to be a hero instead.

Mind, it's not perfect, and has only gotten less accurate as time has gone on, but it's still one of those things that makes Rider, well, Rider.

Ya, it certainly zig zags depending on the series. While some shows, like Fourze, seem only loosely related, other shows embrace this. I still remember how in early Drive, Krim said that Drive was capable of causing the same slow down the Roidmudes are capable of. Ex-Aid's also putting a new a new spin on it as well.

It's like x-men. Mutants are seen as freaks while the avengers are seen as heroes. Civilians used to afraid of riders and they usually hide their identity too and sentai are always seen as righteous heroes. Sentai doesn't question themselves as much as rider do on what they're doing is right or wrong.

I was actually talking more about the origins of the Bugsters themselves, though that is correct as well.