What follows is a more indepth description of the four Origins available to your character. While it's perfectly acceptable to add shades of one origin to another to give you some color (BLACK and Ryuki are obviously Mystics, but with a technological slant, while Amazon was a Bio Rider who got his powers from an ancient gemstone a la Mystic), please keep things largely in one overall style. If you absolutely HAVE to play a hybrid origin...well, that's what kaizou-ningen is there for.
Tech Rider: Your Rider powers come from an assortment of highly-advanced technical devices, including but not limited to a badass suit of armor that ramps up your physical capabilities. You are almost certainly part of a larger organization that both guides and supports you, as well as gives you access to new and better equipment as you need (and earn) it. Plus, since your powers are all based entirely around the tools you wear, you are perfectly capable of interacting with society as a whole, and are most likely to be able to enjoy the simpler pleasures of life as a result.
The major downside to this origin is your complete and total dependence on your equipment in a fight. Should you ever lose access to it - either by having it stolen, damaged, or repossesed by your employer - you will be essentially helpless. Plus, if you are part of a larger organization, you'll be expected to follow their orders and agendas to the letter...which can be a problem if they start asking you to do things that you find morally reprehensible.
Examples of Tech Riders are Riderman, Kamen Rider G3, and the various Riders from Kamen Rider 555 and Kamen Rider Kabuto.
Bio Rider: Rather than gaining access to some kind of device or something that allows you to assume your henshined state, you actually MUTATE into a fearsome creature with tremendous physical power. In Rider form you are fantastically strong and tough, and you have access to weaponry that is an outgrowth of your physical form: ie claws, fangs, pincers, spines, etc. You also probably have enhanced senses and a fast rate of healing, depending on what your Rider form is based upon. But best of all, YOU can never be caught without your powers. No matter what happens, you can always assume henshined state as long as your are conscious, making you a near-constant threat to your enemies that must not be underestimated.
On the negative side, there is no way around the fact that you are NOT human any longer...if indeed you ever WERE human. Being a Kamen Rider means that you are always a step apart from the rest of the world anyway, but you are a good two or three steps beyond that. You are never comfortable around other people, either because you don't understand them, or because you fear what they might do to you if they knew what you really were...or what you might do to them.
Examples of Bio Riders are Kamen Rider Amazon, Kamen Rider Gills, and Shin Kamen Rider.
Mystic Rider: At some point you found or were given an artifact of some sort that transformed you into a Rider and granted you fantastic powers. Whether as a result of magic, psychic power, or a pact with some ancient creature of legend, you possess many wonderous new abilities, some of which may not even be limited by your henshined state. You possess tremendous versatility, and you almost ALWAYS have a trick or two you can pull on unsuspecting attackers.
The key note to be made about this origin is the mysterious nature of your powers. Simply put, most of the time you don't know any more about them than your enemies do (or worse yet, you know LESS about them than your enemies), and that means that often you won't know you can do something until it's almost too late. On top of that, you are dependent on a mystic item in order to access your powers, and while it might be easier to conceal than a Tech Rider's suit of armor, if you lose it you are in for a WORLD of trouble...especially since YOUR item is guaranteed to be much harder to replace.
Examples of Mystic Riders are Kamen Rider BLACK/RX, Kamen Rider Kuuga, Kamen Rider Agito, and the various Oni Riders from Kamen Rider Hibiki.
Kaizou-Ningen: Translated as "altered" or "remodled human", you are a product of science gone wrong. Someone or something decided to use you as an experiment to create a powerful warrior, and as a result your body was tampered with in more ways than one. In many ways you possess the advantages of all three previous origins: you still look human on the surface, but you always have great strength, even when not henshined. You also have access to numerous useful tools and abilities in your henshined form, and the scope of your powers is really only limited by the imaginations and abilities of whoever did this to you.
Sadly, just as they possess the advantages of all three Rider types, kaizou-ningen also possess all of their weaknesses. Suddenly being able to punch a moving semi is all well and good, but what happens when you try to push some little girl out of the way of that same semi? Your body is unnatural now, and you will experience the same disconnection with your fellow man as a result. Furthermore, the experiments done to you have left their own unique mark on your body as well: ranging anywhere from mental instability, to physical rejection of your new implants, to much worse. And let's not forget that whomever it was that made you is still out there, and they probably aren't too happy with the fact that you aren't under their control anymore.
Most all of the classic Riders from Hopper 1 to ZX embody the concept of a kaizou-ningen.