When most people think of Kamen Rider(of those who are aware of this particular Japanese superhero franchise) they think of bug-themed heroes riding bikes, usually with some funky belt gadget linked to transforming them into their superhero form. The bike portion has fallen way off in the past decade or two since Japan passed some laws about custom bikes on public roads, but the attempt is usually there. That's all true enough, but there's one aspect of Kamen Rider that often gets overlooked, but once you know to look for it it's always there in some fashion, and has been since the beginning. Specifically, the rider's powers coming from the same source as the enemy. Or to be even more specific: becoming a monster, but retaining your humanity to fight other monsters.

In the early days, cyborgs were a big part of the franchise, and the titular riders were modified into cyborgs to become living weapons. That specific implementation is rare these days, as the everyman is well aware that having body parts replaced doesn't turn you into a Shadowrun cyberzombie, but the spirit is there, even today. To give some examples(and warning, spoilers are ahead, as these are often plot points too):

• In Kamen Rider Faiz, the transformation item is clearly a mechanical augmentation. However, you find out that only Orphenochs, who are basically undead humans that can take a battle form, can use these, or those with Orphenoch modifications. And the Orphenochs are who the Riders in this series fight to protect the public from.
• In Kamen Rider Blade, Kenzaki starts off as a normal human, fighting Undead(who despite the name are actually just a race of immortals) with equipment powered using Undead that are sealed in cards. By the end of the series, through his high compatibility and constant use, he is transformed into one himself. Kamen Rider Gaim has a similar series of events.
• In Kamen Rider Ex-Aid, most of the riders had to have "compatibility" surgeries to use their transformation belts, but the main character is able to use it without one. It's eventually revealed that he was infected long ago with the virus that the doctors are "treating"(aka fighting), and this is why he can use the driver.

To be fair, some series don't actually change the person themself anymore, but they still use the same source:

• Kamen Rider Zero-One's heroes are powered via Progrise Keys, which are developed by a super-AI, and so are their opponents, who usually are linked in some way to another super-AI that has decided to wipe out humanity due to it's observations of human malice.
• Kamen Rider Kabuto never turns the main rider non-human, but the Zecters that the belts use to let them Clock Up(think super-fast movement and reactions) and fight the Worm on even ground were developed from said Worms.
• Kamen Rider Drive's Shift Cars and Roidmudes(basically battle androids with some sort of space/time manipulation and noncoporeal true form) use the same underlying technology

Ishinomori, who created the concept, very much was behind this idea of a "Monster who fights monsters", but it is far more evident in the manga he created, as outside of the old Shin Kamen Rider(not the new one Anno created) the rough edges were filed off for the sake of the TV shows.

• In the original Kamen Rider, Kamen Rider #1 and #2 are merely grasshopper cyborgs to Shocker, and other than keeping their human selves, they're not really any different in scope from, say the bat cyborg or the spider one. Because of this, at one point they have to fihgt a group of grasshopper cyborgs who look just like them, but did not escape the brainwashing like the first two did.
• In Kamen Rider Black, a portion of this actually did make it to the TV show, though it's not highlighted. He has a more monstrous grasshopper-person form and the black suit is armor over top of that - when he transforms in the show, for a split second you can see the original bugman form before the armor forms around it, and the material in the caps in the armor are supposed to be showing the joints and muscles of the bugman underneath. In the newer Kamen Rider Black Sun adaption by Amazon, they actually make greater use of this form.
• Lastly, the old Shin Kamen Rider is fully biological, and his transformed form is very organic grasshopper-person. This was due to Ishinomori being allowed to lean harder into his vision for this specific movie, though it never was turned into a show.

This is a concept I find intriguing, and have played into a few times with my various characters. A couple examples from my profiles here:

• Lunaria is the most obvious example, as a holy werewolf protecting the people, while they're probably scared to hell of the big buff werewolf that they think might eat their face.
• Sharilar is powered by eldritch beings, and even becoming one herself, yet will go out of her way to fight their cultists and make sure no others are corrupted in the way she was. (And in fact, drew a lot of inspiration directly from Kamen Rider Wizard, who also awakened to his magic via being a victim to a cult sacrifice, though in his case it was under an eclipse).

Now becoming a monster to fight other monsters is not something unique to Kamen Rider by any means, nor do I think it was the first. It's tied to the franchise's very DNA however, and is the most consistent example I can think of across the nearly 50 years or so of its existence. Most of the other possible examples I'm aware of in the mainstream do not have the villains or antagonists using the same abilities and power source across the board. But given that most people focus on the obvious aspects of the franchise and overlook the shared power source(outside of the full Rider vs Rider seasons), given how intriguing and fun that very concept is to play with, I felt it was worth bringing up.