It could also be because trains and grasshoppers are fast with a hard exterior, fitting how most Riders have a Jack of All Stats base form.
Trains are also very common and popular in Japan, there are even train aficionados. One could say it's as popular as Kamen Riders.
Both are also very powerful, and in combination can achieve greater success, which is one of the core aspects of alchemy, taking things and making them better.
Houtarou's name also is based off of first Kamen Rider and that of Den-O. His surname is Ichinose like Kamen Rider Ichigo. And the protagonist of Den-O was Ryotaro, which sounds similar to Hotaro.
Many steam trains may contain hopper cars to help carry cargo while grasshoppers are a near homonym for this kind of train car.
The non-humanoid Wild Forms are a transmutation that occurs when only the chemies are in sync. This means wild forms are more of a simple transmutation of two chemies. The reason the normal forms are humanoid? When Hotaro is able to synchronize with the chemies, he himself is literally added to the mix, forming a perfect three-way transmutation.
If you look at the belt from an above angle, it looks like a bento box. This is significant for two reasons:
Bento boxes are common lunch foods found in schools. Plus Houtarou's mother owns a restaurant that specializes in bentos.
Bento boxes are all about mixing and matching different foods in one convenient package, like how Gotchard's forms are about fusing different items together. And how is Houtarou introduced in the show?
The reason Den-O and Zero-One are a match? Because it's the Legend Rider equivalent to SteamHopper, with Den-O being a train based Rider, and Zero-One being the first Rider since Ichigo himself to have a grasshopper motif and be the start of a new era for Kamen Rider. A train with Liner in its name, and a 1st Hopper, get it? This is outright confirmed to be the case by Bandai/Toei with variant cards which have the Riders missing, and the Legend Chemies in the same poses as Hopper1 and Steamliner. The other Rider matches also have their reasons:
Kuuga/Faiz: Both are the first in a number of categories, the former being the first Heisei series and the latter being the first to use a purely technological power set.
Agito/Blade: Aside from both Riders having a beetle as part of their motifs (Agito's being partially based on a stag beetle and Blade's being a Hercules Beetle), they both fought deadly mystical beings based on animals.
Ryuki/Ghost: Death is a prominent theme in both series.
Hibiki/Ex-Aid: Music and Oni have been featured heavily in video games.
Kabuto/Saber: Time is an element of both series.
Kiva/Wizard: Vampires and dragons are both mythical beings. Coincidentally, both Riders also shared a consecutive Rider Kick in Heisei Generations Forever.
Fourze/Build: Both series have a technology and space motif.
Decade/Zi-O: Both are anniversary series for the Heisei era.
Double/OOO and Gaim/Drive: OOO and Drive followed Double and Gaim, respectively.
Revi/Vice: Both are the main protagonist Riders of their series.
Geats Riders: It's very straightforwardly the main four Riders of the series paired with their respective supporters' Rider forms. Glare/Gazer is the only different pairing, though that's only because they're both DGP staff Riders and they use the same Transformation Trinkets and suit bases.
Houtarou is known to help his mom at his restaurant. And what is cooking but its own type of alchemy in mixing things together?
While some of the reasons for SteamHopper's existence are listed above, here are a few reasons for the others.
Apparebushido + Skebows: Both samurai and skateboarders require intense focus and body control to pull off their skills.
Antrooper + Wrestler G: Both are ridiculously strong and can work individually or as a group.
Also, both deal with grapples and throws; both are staple moves in the world of wrestling, and ants are better known for lifting up to 5,000 times their own body weight rather than attacking through brute force.
Hawkstar + Saboneedle: Both hawks and cacti are known for having sharp and pointy body parts.
In addition, some species of hawk live in the desert like cacti do.
Mechanichani + Golddash: Both crabs and motorcycles have tough bodies and high mobility.
Raidenji + Junglejan: The phrases "Lightning never strikes the same place twice" and "It's a jungle out there!", both of which correlate to unpredictable outcomes. Furthermore, certain batteries were inspired by the electric eel, an animal that lives in the rivers of the Amazon jungle.
Deepmariner + Venomdake: The most common anchors for submarines are mushroom anchors. Also, both have significantly more of themselves underwater and underground respectively than you can see from above. Furthermore, most mushrooms prefer dark and damp areas out of direct sunlight, while most submarines work underwater, particularly at depths that get very little sunlight.
Gorillasensei + Burningnero: This combo may be a homage to FireKong from Kamen Rider Super-1 as both are gorilla-themed with fire-related powers.
In addition, both are known for their strength, gorillas in terms of their muscle capacity, and habaneros in terms of how spicy they are.
Dokkirimajin + Gutsshovel: A lot of card tricks used by magicians involve hiding cards up their arms while excavators are famous for their shovel arms.
Hiikescue + Flayrose: Both fire engines and roses share the same colors, and are associated with toughness and water.
Odorippa + Kamantis: Praying mantises dance to locate prey and males also dance to attract mates.
Dancers also tend to use blades in combat to pair with their stylish moves. Built-in ones from a mantis would be perfect.
Energyl + Sasukemaru: It's said that ninjas would eat hyōrōgan to help endure the harshness of missions and give them an energy boost, kinda like how an energy drink would.
Pilets + Madwheel: Pirates are known for pillaging. Cars can be used for street racing. Not only are both illegal but putting both together gets a high-speed raid.
Gengenchouchou + Bulletbang: Both butterflies/moths and firearms come in many different varieties, and both the insects themselves and fired ammunition, are capable of being airborne.
Smaphone + Pikahotaru: Fireflies use their light for communication as we do with smartphones, and smartphones often have flashlights.
Spicle + Ganvhale: For both bikes and whales, larger variations move much slower than the smaller ones.
Yamibat + Renkingrobo: Bats and robots have been a common element in most superhero media, and there have been several cases before Gotchard where both of them have been combined.
Bakuonzemi + Televi: Both cicadas and TVs are known to be loud.
Stagvine + Mitemirror: This could be a reference to Arata Kagami of Kamen Rider Kabuto, as he was primarily a stag-beetle-themed rider whose last name is a homophone for the Japanese word for mirror (鏡).
Greatonbo + Bussasorry: Dragonflies and Scorpions have a lot in common. Aside from both being similarly shaped arthropods, both symbolize transformation and have been seen as signs of evil in various cultures. Finally, both are predatory arthropods that feed on other arthropods.
Panpakaparka + Bountybunny: Parkas are famous for keeping the wearer warm, similar to a bunny's fur coat.
Catchula + Tricera: Spiders that actively hunt tend to charge toward their prey, while Triceratops are famous for rhino-like charge attacks.
Happyclover + Ninetail: Both four-leaf clovers and nine-tailed foxes are said to be signs of good luck.
Zukyunpire + Sabeliger: Both vampires and sabertooth cats are known for their long canine teeth or fangs.
Angelead + Warptera: Both angels and pterosaurs are known to be flight-capable.
Bambamboo + MacKraken: Both bamboo plants and squids/octopi are stereotypically associated with long-range jab attacks due to their length.
Kesuzo + Jamatanoorochi
The original Yamato no Orochi was slain by being decapitated and hacked to pieces. Kesuzo (an eraser) is represented in KesuOrochi's jingle by the music skipping and scratching out as if it were playing from a scratched disk or tape reel.
Alternatively, the segmented appearance of KesuOrochi's helmet caused by Kesuzo's influence more directly represents the dragon's demise.
A wordplay with head and its count—with Orochi, Gotchard has the power of 8 eraser heads!
Both are associated with the color red (the Yamata no Orochi having a red belly and erasers typically coming in light red) and have an association with trees (rubber erasers coming from rubber trees while the Yamata no Orochi was said to have fir and cypress trees growing off of its back).
Sayzombie + Gigalodon: Zombies are typically reanimated corpses, while most prehistoric creatures have fossilized remains; both involve death before their current states. In addition, zombies can multiply by biting down on healthy living organisms while megalodons are an extinct species of shark, an animal known for its bite.
Kaiserbee + Akumanocharis: Both bees and anomalocaridids are often mistaken for attacking various creatures that they would otherwise have no interest in approaching unless threatened (humans for bees, trilobites for anomalocaridids.)
Utsubocchama + Ojilacanth: Pitcher plants and coelacanths both feed by simply waiting for other creatures to approach their maws.
Gekiocopter + Daiohni: Helicopters have become infamous for being notoriously lethal against ground infantry, and despite their underworld themes most Oni are associated with beings from the heavens. Both correlate to the phrase "Death from Above".
Buglesia + Berosol: Rafflesia flowers rely on a foul stench to attract its only known pollinator: flies. Meanwhile, Kasa-bakes are notorious for licking people by surprise, which many would consider a foul act.
Timelord + Pakuraptor: Dinosaurs are famously known as creatures from a lost time.
Why a Velociraptor in particular? It's a bit of a stretch, but the velociraptor is supposedly built for speed. In other words, it evolved to be able to cross great distances in a short amount of time. And what's great at telling and measuring time? Why clocks and watches?
Karyudos + Nammonite: Hunters tend to wear armor to blend in with their surroundings when looking for their target, while for ammonites, their shells are the armor, and were likely used to hide from predators.
Carery + Blizzammoth: Mammoths are famously ice age creatures, and ice itself is not only used in certain medical treatments but is also one of many Elemental Motifs used by many fairies. Furthermore, fairies are said to be gentle while mammoths are the extinct counterpart of elephants, an animal typically associated with being a Gentle Giant .
Like Gotchard, Majade's Chemy combos have a theme.
Unicon + The Sun: Starlight is a common motif associated with modern portrayals of unicorns, and the sun just so happens to be a star. Also, both the sun and the unicorn are associated with purity and divinity which are reflected in Majade's white armor and abilities.
Neminemoon + Yoacerberus: Both our moon and Cerberus/Kerberos are natural satellites of planets (Kerberos orbiting Pluto), and the Japanese word for "moon" (月) is both a homophone and homonym for that term.
Kuroana + Gigabaham: Kuroana, a black hole, is an harbinger of destruction while Bahamut, a mythical creature said to have held aloft the universe, represents preservation with its dragon portrayal having the capacity to end the world. Both together represent creation and destruction, which are the foundation of alchemy's principles.
Gaiard represents the four Natural Elements of alchemy, while Dragonalos is a being made out of rainbow crystals. They are both representative of symbols commonly associated with All Your Powers Combined.
Jupitta + Makentauros: Jupiter is known in Japanese as the "Wood Star" (木星). Much like wood by itself, centaurs symbolically represent a halfway between wild nature and manmade structures.
Grandsaturn + Vanfenrir:
Fenrir is the name of a mythical wolf, and also the name of one of Saturn's moons. Like Kuroana and Gigabaham, they are also opposites astrologically: while Saturn is a massive, brightly-colored gas giant, Fenrir is a tiny asteroid and one of the darkest-colored objects in Sol, so dark that even modern probes fail to capture its presence at times.
Alternatively, Saturn is known in Japanese as the "Earth Star" (土星). The mythological Fenrir, a wolf, would have the closest representation of the element among those represented by the Fantastic Chemies.
In their physical appearances, Grandsaturn's actual body is the ring of rock surrounding its caricature of the planet, while Vanfenrir is covered in ice. This bring to mind permafrost, frozen soil that has remained frozen for at least several years.
Firemars + Inphoenix: Not only is Mars known for being red like fire, but the Japanese word for Mars, Kasei (火星) contains the kanji for fire (火). Phoenixes are typically associated with fire.
Mercurine + Donposeidon: The Japanese word for Mercury, Suisei (水星), contains the kanji for water (水), while Poseidon was the Greek god of water and the sea.
Gokigenmeteor + Haodin: Comets and meteors used to be regarded as the "swords of the gods" (this is where the word "disaster" comes from), and Odin was a god of war.
Kinkiravenus + Gingriffon: Griffons were said to guard gold and treasures. Not only is Venus a golden-yellow color when viewed from space, but Venus is known in Japan as "Kinsei (金星)", which translates to "gold star".
The naming scheme for these two Chemies might be a nod to "gold and silver".
When Supana previously used Valvarad's initial form, he solely uses Vehicle Chemies in relation to Valvarad's mechanical appearance. It also tied directly with Supana's mindset that Chemies are merely tools for humans to use. While you could argue that the Chemies based on living beings could technically still be alive in some sense, vehicles are still inanimate objects that require someone to operate them in some way. But after becoming a Kamen Rider, Valvarad follows Gotchard's example by using non-Vehicle Chemies. Some of these Chemies also have a personal relationship to Supana.
Madwheel/Machwheel + Daiohni: Cars and the supernatural have intermixed before, and given Oni are comparable to demons in western mythology, the term "Speed Demon" comes to mind. Madwheel was also Valvarad's primary Chemy and used in his pre-Kamen Rider form.
Gutsshovel + Jyamatonoorochi: There are excavators with more than one arm, much like the multi-headed Yamata no Orochi.
Gekiocopter + Angelead: Both helicopters and angels are famous for their flight characteristics. Angelead is used to create the Angel Malgam, whose actions restore Supana's memories and forced the events that lead him to become a Kamen Rider.
Spicle + Carery: Bicycles and fairies are classic representations of childhood innocence, and both have parts that come in pairs (most notably, a bike's wheels and a fairy's wings).
Skebows + Berosol: In line with the Kasa-bake's foul reputation, skateboarders tend to have a poor reputation amongst the general public.
Hiikescue + Sayzombie: In a particularly dark example, numerous people lost their lives saving others, firefighters among them.
Deepmariner + Zukyunpire: Submarines operate deep underwater in areas with little light, while vampires are famously creatures of the night.
Golddash + Mckraken: Some motorcycles have V8 engines, and the exhaust pipes for these rides look like a mess of tentacles.
Steamliner + Ninetail: The age of steam for railroads had ended in the 1900s, with rising beliefs in legendary creatures such as kitsune also around the time.
Exceedfighter + UFO-X: A number of UFO sightings were from fighter pilots on a training exercise.
To pass the test and join the Alchemists' Academy, Houtarou is tasked to make the heavy Odorippa and light Kamantis weigh equally, and he succeeds in this by simply asking both Chemies to change their diet. For an alchemist, this would be too absurd to be such a simple solution, but it works for him because his family deals with boxed lunches, as such he uses the knowledge that one's diet is important for weight and energy control.
Both Houtarou and Supana represent the two most common sides of the sentient machinery depictions; Houtarou is willing to treat the Chemies as equals and sees them as actual living beings, and as such, Gotchard's forms are a perfectly harmonious blend of biotic and abiotic elements, representing the more humane depictions of living machines. By contrast, Supana sees the Chemies as nothing but powerful but insignificant tools, with Valvarad using solely mechanical design elements. This results in a chaotic patchwork look with additional option parts tacked on, representing the heartless machine stereotype.
Each of the non-Dark Sister/Malice Doll Malgam hosts has a characteristic that connects them to their respective Chemy.
Sudo needed a way to escape the police chasing him after a robbery, so a skateboard like Skebows could give him the speed and mobility to do so.
Takemoto not only resembled a mushroom due to his bowl haircut, but his hate for other people also made him a very toxic man.
Kariya was Rinne's creepy stalker, and Deepmariner's swimming abilities would allow him to follow her undetected.
Gouriki the Golem was a big, imposing man with a build akin to that of a gorilla.
Despite not turning into a Malgam due to Supana's timely intervention, the girl in #6 had a burning hatred for the guy who dumped her.
Boruto sought to prove his superiority over Supana and the rest of the Academy, that is, he wanted to soar over them.
Kenji became a dragonfly hovering over his son to make sure he focused on his studies like a helicopter parent.
Yabuki's goal during his debut was to escape the law. The jungle would be the perfect place to achieve such a goal.
Tsurugi was an arsonist. In other words, he loved to send sparks flying, whether it'd be fiery sparks from a lighter, or electric sparks from a battery.
Hijiri became the second Battery Malgam after Tsurugi's defeat. Some batteries are rechargeable, not to mention that she reacted to a rather shocking reveal.
Shiori Harima knows how to put the "spy" in "spider".
Licht was hungry for enough power to permanently defeat Ace, so of course he chose X Wizard, the most powerful of the Chemies.
The Orochi Malgam has Geryon possessing Lachesis as its host, making it multiple minds in a single body.
Supana had a bunch of repressed trauma from Geryon killing his parents. When it finally happened again, he was driven mad like Madwheel, and he was roaring like a car's engine.
The two unnamed land-grabbers wanted to wreak havoc over Kokonotsu just like Berosol and Carery, but with far more malicious motives.
The fox spirit watching over Kokonotsu became a Malgam not because it was malicious, but because it wanted to trick Houtaro into uncovering a hot spring for the benefit of the village. In other words, it used trickery by pretending to be a villain to do a good thing.
The unnamed criminal from #32 wore black clothes, just like Akumanocharis. Also, "akuma" (which is part of the Chemy's name) means "devil" in Japanese, which is not a person anyone wants to approach, like how nobody approaches a criminal. The criminal in question also wanted to evade arrest without causing trouble, only going physical if anyone (such as Sabimaru and Renge) tries to stop him.
Chemies absorbed by malice are devoured by a monstrous maw-like construct, reflecting the reckless desire to consume without care, or to indulge in excess, as if consuming food or beverage. Inversely, people who have a good synergy with them will instead wear them on the outside as if they were a costume or clothes.
Cacti are famous for surviving in notoriously harsh environments and blooming once favorable conditions show up. This mirrors Riku's lifestyle; his father's oppressive treatment is barely tolerable at minimum and insufferable at worst, and once Riku is allowed to take a break from studying he is much happier as a result.
It's stated by Lachesis that most humans can't do multiple alchemy, as they can't handle multiple Chemies at once. Lachesis and Supana seemingly can do so because of their skill and using Chemies of the same category, equipping a tool formed from one Chemy to a base formed from another. This contrasts with Houtarou's use of two Chemies of different categories to create a perfect combination. In chemistry terms, this could be compared to the difference between a heterogenous mixture and a homogenous solution.
Kamen Rider Legend is the exact opposite of Kamen Rider Zein, other than their powers being derived from Kamen Rider Decade, both Kaguya and Zein all embody Tsukasa Kadoya's positive and negative traits, respectively:
Kaguya essentially has Tsukasa's bravado and kindness underneath his overwhelming pride, while he uses his powers for altruism. Legend has his own set of Chemy Cards themed after previous Riders, in addition to transforming into Riders like his template, he can also summon Riders like Diend.
Zein, on the other hand, is basically what Tsukasa would have been like if he never lost his memories of his time as the Great Leader of Dai-Shocker and could have used his powers to conquer A.R. worlds, taking his role as the "Destroyer of Worlds" more literally and figuratively. The Zein Cards are artificial knockoffs of Decade's Rider Cards, while it doesn't allow Zein to transform into a Rider but can replicate the Finishing Moves of Kamen Riders in their Super Mode, albeit can be only used once.
Some Chemies have additional features that further hint at their compatibilities:
Goldash has robotic claws that resemble a crab.
The way Yamibat emerges projects a shadow that can go as big as Renkingrobo.
Greatonbo can shoot projectiles from its tail. Its tail can be used as a weapon like Bussasorry's stinger tail.
Why is it, Supana notes that when fighting Kamen Rider Dread's fighting style has gotten dull since the first time it appeared? Well actually it was due to three factors. 1) Sabimaru could've been fighting back from the inside trying not to hurt anyone else long enough to be freed. 2) The Dark Sisters stated that he wouldn't last long attributing to the fact the Driver drains the user's life force. So it being dull could've been attributed to the driver weakening the user. 3) While it is strong, the user isn't exactly a trained fighter, so while he did overwhelm everyone in his debut, it could've been attributed to the Driver giving him a boost.
This is actually confirmed when one of the sisters uses the Driver and are able to defeat Supana in a matter of a few moves.
Of course the Chemy that resonates with Rinne is a unicorn. After all, unicorns were known to approach pure-hearted maidens! At this point, Rinne has let go of her assumptions of her father being a bad guy, thus proving to Unicorn Chemy who has finally decided to show their face to her.
Fire Gotchard has yellow flames over blue armor panels. Anyone who's seen a gas flame will know that the yellow flames are always blue at the bottom, where the heat is most intense.
Gotchard Daybreak has a warmer color scheme compared to the normal Gotchard, but he has a colder demeanor despite his colors. This makes sense for two reasons:
As mentioned above, blue flames give more intense heat readings than red or yellow flames, matching the normal Gotchard's fiery spirit over Gotchard Daybreak's more mellow disposition.
More people are active and energized in the middle of the day compared to early in the morning and late at night when the sun sets and rises. This is represented by regular Gotchard firing on all cylinders while Gotchard Daybreak dials it back significantly.
The first set of lyrics for CHEMYxSTORY fits Hotaro's desires to change the world and defy people's expectations. The second set focuses on Rinne deciding to stop following the rules of the Academy and do what she thinks is right, stating that she's not alone in her endeavors, along with Supana learning to open his heart up to others.
Why didn't Kyoka simply get Spanner to a therapist? Because that would mean revealing the alchemist society to the outside world, and given how Spanner could project dangerous spikes it would both hamper any attempts to get close to him to try anything and make upholding the ruse almost impossible.
Supana getting a purple Alchemist ring isn't just because purple is his signature color. Purple is a mixture of red and blue. The red is from his parents being high-ranked Alchemists, so part of it is in memory of them. The blue is Hotaro's color as Kamen Rider Gotchard, symbolizing how he's starting to see the Chemies as something past tools. Moreover, blue is the beginner level Alchemy ring, also tying with how he begins developing into a more open warrior.
All three of the primary Riders have a color found in chemical flames, which generally fit around their characters:
Blue flames are the closest to the source and the hottest in terms of temperature, and are generally more precise despite needing special circumstances to be put into use; despite the Gotchardriver's limiting factor, Houtarou's potential using the system is the most potent, and his driver serves as the base for the other two's. Additionally, he has the best people skills of the three, being the most amicable of the characters by far.
Orange flames are further from the source and slightly lower in temperature, but despite this, they do not require special conditions for usage and have a much wider range of utilities; Rinne lacks the sheer power or speed of the boys, but her Chemy selection allows her to adapt more to keep up with them, and is an accomplished alchemist despite being of the most junior ranking possible. And despite not being as friendly as Houtarou, she is much more socially adjusted than Spanner.
Red flames are the furthest from the source and the lowest in terms of temperature, but perform well in their few uses; The nature of the Valvaradriver's systems means that Supana is more limited in utility than the other Riders, but what little abilities he does show are more than able to contribute to the fight. He is very much the least sociable of the three, but he does instill standards for himself.
If you look closely at the details, you can see that Dread's first three types are based on how many horns are on the Chemies and how the Chemies correlate to their users and the Riders associate with the normal counterparts to the Repli-Chemies, with the actual Type 3 representing all three working together.
Type 0: Steamliner; while there are a few that do, there are borderline zero steam locomotives that use horns over whistles and bells. Type 0 was the most commonly fought form, and clashed with Gotchard far more often due Majade and Valvarad appearing later in the game. The users of Type 0 also either were close to Houtarou (Sabimaru and Minato) or bore grudges against him (Clotho and Lachesis).
Type 1: Unicon; Unicorns are famous for a singular horn that protrudes from their scalp. They are almost never depicted with more than one horn. Type 1 was primarily used by Minato and Atropos, both of whom tried to break down Rinne through psychological warfare rather than physical combat.
Type 2: Daiohni; While single-horned variants do exist, it is almost ubiquitous to depict oni with two horns. Type 2 was first used by Geryon, whom Spanner had a personal beef with due to the fact the madman killed his parents. It's later usage by Clotho also represents Spanner's original Can't Catch Up status, as she is unable to use Type 3 without severe consequences.
Type 3 uses all three Repli-Chemy cards. The form is exclusive to Geryon, whom each of the three Kamen Riders has a personal streak against: Houtarou in being an antithesis, Rinne in forcing her father to abandon her, and Spanner as his parents' murderer.
Fuga's element as a Kamen Rider is wind. Of course, he's fanning the flames of a rebellion to stop Geryon.
Why are Gotchard's Legend Rider forms all gold while Legend basically gets a nearly identical copy with each one? Because Legend himself is a Decade fanboy and is more likely to be faithful towards his source of inspiration, as the Decadriver works in exactly that fashion. The Gotchardriver, however, tends to vary on the colors, but since the Legend Rider cards are not the same as normal Ride Chemy Cards, their primary association with Legend causes the end result to appear in his specific signature color (gold).
Of course the gold obsessed Geryon was beaten by Platinum Gotchard! Platinum is worth more than gold.
Each of Platinum Gotchard's combos when defeating Geryon was a Laser-Guided Karma for each aspect of his crimes committed by either him or his pawns.
Saboneedle + Wrestler G: Two Chemies who had bonds with humans that they were forced to sever because of Geryon (and the Academy's) corrupt system.
X-Rex + UFO X: Their appearances coincided with the time that Kamen Rider Dread first appeared, where Sabimaru being the first unwilling user.
Kaiserbee + Mechanikani: Kaiserbee was one of the Chemies captured by the sisters after the Academy subsequently suspended the kids.
X Fortress + Odorippa: X Fortress was one of the Chemies targeted by Licht, a supervisor in the Academy, in which the incident was the excuse for getting Houtaro kicked out of the academy while a test involving Odorippa was what got Houtaro into the academy.
Raidenji + Catchula: Raidenji was the last of the Chemies to be turned into a normal Malgam of the Week while Catchula was used by a pawn to help create Dread's Replichemies.
Renkingrobo + Warptera: Renkingrobo was used to battle the Jungle Malgam, the very same Malgam that was used to distract Fuga while Geryon massacred the school and traumatized Minato into turning on them. Meanwhile, Warptera's powers was what put Houtaro in the Iron Gotchard mess in the first place, and, fittingly enough, is the last of the combos before he pulls off the Finishing Move.
The Dark Sisters and Kamen Rider Legend (Specially Legendary Legend form) have plenty of inspiration from Egyptian attires, but this has an explanation: Alchemy was born in ancient Egypt after all.
The design of Kyubi Malgam doesn't show the Ninetail being abused or twisted in any way, a sign that it is in no way a Malgam made out of malice, but rather of a desire to help the village prosper. But why did it take the form of a Malgam rather than something else? Simple: Gotchard would not appear unless a Malgam was involved. So it was required to be perceived as a serious threat requiring immense power to break through a stone wall and uncover a hot spring.
When Houtarou confronts the Kyubi Malgam, he calmly states how this Malgam is different. Given our hero can sense malice, he must have not sensed any malice coming from this Malgam, and that was yet another hint that this isn't a Malgam proper but rather a Chemy simply letting the local deity borrow its power.
Aside from the obvious Ship Tease angle, it's easy to see how Rinne lands the role of Juliet in the School Play in #31. After all, one of the famous lines of Romeo and Juliet is "It is the east, and Juliet is the sun!" Now, which character is most associated with The Sun Chemy again?
The Ark-One Malgam's choice of Chemies to fuse with fits well with the fact that Zetsumerise Keys contain data of prehistoric animals, as their unearthed remains are the only thing left to discover on the present day. Nanmonite and Gigalodon are extinct, Sayzombie, as its name suggests, refer to zombies rising from the ground, and Gokigenmeteor refers to how meteorites are also unearthed specimens like extinct animals.
Zein's murder of Mimei, a general of Hundred that is bent on interdimensional conquest, is basically a metaphor of shooting trespassers on sight. To Zein, the world of Gotchard is the next Rider world it seeks to annex into the merged Rider worlds in order for it to rule, viewing the presence of Hundred as the intruders entering its domain.
Why Atropos and Gigist made the decision to stay out of the conflict in episode 33 while allowing Clotho Rebis to fight? Because they cannot risk fighting a superpowered dictator head-on, not to mention that Zein has the ability to place the entire world under strict surveillance. Even its Rider form comes equipped with various surveillance devices that allows it to control spy satellites and security cameras, and a lie detector within its suit. If Zein managed to find and kill Gigist and the two Dark Sisters, Gotchard might have abruptly ended with a short of 33 episodes.
A more likely possibility comes from them having more important matters to attend to, only sending Clotho Rebis because there is a very likely chance she might side with the invaders and help dispose of the heroes.
Take a good look at some Malgams like Gorilla Malgam and the Cerberus Malgam. In the case of former, its head and shoulders aren't just modeled after a gorilla, that's Gorillasensei trying to stop the Malgam and failing. In fact, many Malgams are shown to be restraining the Chemies they absorbed.
Episode 6 shows two horrifying revelations. 1) even students from the Alchemist Academy can be seen as targets for Malgams. And 2) They are also equally targeted for the memory wipe. That begs the question how many personnel have had their memories wiped by them?
Supana did not recognize Boruto at all even though it's only been one year since they had their battle. Has he had his memories constantly wiped as well? If he has, he's nothing more than a tool or vehicle that the higher-ups are using for their own schemes.
Episode 9 had a kaiju battle in Kyoto where thousands of people watched. Not only will Minato and other alchemists have a hard time wiping their memories, but how many of those onlookers uploaded the video on social media? How many millions more people around the world are going to have their minds wiped?
For that matter, how many people had to get wiped from the Chemy invasion in episode 1?
The reason why Saboneedle detaching itself from the Driver is never explained, but most likely Saboneedle sensed that the Dragonfly Malgam was Riku's father. Hotaro was probably going to injure Kenji badly without understanding what was going on.
It is more likely that Saboneedle thought that Riku's father could get badly hurt or even die, and the thought of that happening scared him to the point he needed to cool down.
#13 has a random teacher being killed in broad daylight by Atropos. Did that teacher have a family or friends that need their memories wiped? The trend continues in #17 with Minato, given that only a handful of people remembers that he exists.
If Minato is really The Mole, there's something saddening on how he's risking his life every time he becomes Kamen Rider Dread and doesn't care that he's closer to dying if it means getting to the truth.
Given how Supana reacted to remembering his parents' death in #20, it's a really good thing X-Rex didn't devour him to get a look of his memories.
While it's nice that Yua remembered Zukyumpire's words in #23, there is a sad moment that sooner or later, the Academy might make her forget all about the encounter.
If Zukyumpire never learned of the consequence of his abilities, he would have very well drained the life energy of everyone on Earth with his ultimate Zukyun.
#24: The actions were a Morton's Fork for Minato — if he doesn't get the Chemies, then Geryon would send someone else to slaughter his students. If he does get the Chemies, he spares his students but he's only delaying their demise when the Door of Darkness opens.
The actions of the Three Dark Sisters after Geryon's defeat evoke that of children who grew up in an abusive household.
Atropos mimics her "father's" sadistic side, proving to become just as big an abuser as he was and maintaining the cycle of abuse.
Lachesis attempts to escape and defects to the Alchemist's Academy, though it's also clear that her trauma will take some time to heal.
Clotho serves as the conflicted sister who wants her family to stay together, even as it becomes clear that their relationship will never be the same again.
How did Zein got roped into the show's conflict? Remember that Zein has the entire world under surveillance since its introduction. The presence of both Gigist and Hundred forced Zein into the fray. However, Houtaro and the others should never ever ally with Zein considering that it's proven to be far more dangerous than Geryon and Gigist, considering its actions in Outsiders.
