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Akazukin Chacha: a little unusual in that there were actually three Transformation Trinkets— shared among the three leads— which had to be combined for Chacha to transform.
Cutie Honey has her choker. In Cutey Honey Flash, she has a matching ring, and in Cutie Honey: The Live, her trinket is a pendant that turns into the classic choker when she transforms.
Sailor Moon: Usagi and Chibiusa used jeweled brooches, while the other Solar-System Senshi used wands. Usagi also had a Disguise Pen (which was used in the first two seasons of the anime, but dropped after that). The Sailor Starlights used head-mounted microphones, and in the live action series, as well as their transformation items (now bracelets), the girls had cellphones which they could use to disguise themselves. Heck, at one point Sailor Moon even used the Holy Grail to become Super Sailor Moon... or at least something called that for a while, then officially named the Cálice.
In Tokyo Mew Mew, the girls didn't originally need transformation pendants, but they were added for the anime. Unlike how they are portrayed in the former page image, the pendants were only part of chokers while transformed, not before, when they were just carried around like a normal Transformation Trinket. Just for the record.
Ojamajo Doremi uses the "taps", circular musical instruments which vaguely resemble tambourines. In the second season, they were replaced by similar-looking devices shaped like flowers. In the third season, they used circular wrist-mounted devices that were attached to rings. Finally, in the fourth season, they use the "Cologne Taps": perfume bottles which, like the HeartCatch Pretty Cure! example above, trigger the transformations with a couple of spritzes; it doesn't help that both series have the same character designer.
And while we're at it, Hana-chan had a transformation device that resembled a compact and was activated through the power of the other girls.
In Mahou Sensei Negima!, anyone can form a contract with a mage to draw on their power. This contract produces a tarot card, which can be used, among other things, to summon a weapon (sometimes extremely unusual) and a variety of user-defined costumes.
In Bleach, after the Soul Society Arc, Ichigo is given a pendant that ejects his shinigami soul from his body so he can fight monsters. It beats the hell out of his earlier Transformation Trinket, which is a glove Rukia wears while she punches him in the face; and Kon, a modified soul inside a candy that displaces his own. Although Kon is still somewhat useful, because Ichigo's body is essentially dead when Ichigo leaves it.
When Rukia is in the real world she uses her own mod soul called "Chappy", which has an irritatingly cute personality (this was what she apparently tried to obtain when Ichigo ended up with Kon).
Guy Shishioh from GaoGaiGar could in fact be considered the trinket himself, as he does technically 1) As cyborg guy have the power batteries that make him go hyper mode integrated into his body, where as 2) Being Evoluder Guy, he literally becomes part of the machines he pilots.
The pocket watch in Chrono Crusade could be considered to be a very untraditional Transformation Trinket. When Chrono opens the watch, it allows him to turn into his true form—but the watch is what keeps him looking like a young normally. It doesn't make him transform so much as it removes the seal on his powers.
Princess Tutu has a pendant for the main character, Ahiru. However, it's not just so she can turn into Princess Tutu—it's also so she can turn into a girl. She's really a duck.
Not only that, but it also turns out to be one of the pieces of Mytho's heart, the feeling of hope.
The Millennium Ring also hosted an ancient egyptian spirit who could could take control of his descendant's body when wearing it. The third alter ego, Yami Marik completely averts this trend, though the normal Marik was able to go into the Millenium Ring, sharing a body with both Yami and Normal Bakura, but not able to physically manifest himself like the two spirits of the millennium items are.
Umi Monogatari has the Bermuda Stones, which turn Marin and Kanon into priestesses. Which makes it a bad thing when Sedna destroys them.
Fate/kaleid liner PRISMA☆ILLYA: The Kaleidosticks allow its users to enter a Magical Girl trance, granting them access to an unlimited supply of mana. The Class Cards also serve a similar function when installed, granting the user access to the powers of the Heroic Spirit it is based off of.
Donald Blake's cane allows him to transform back to his true form, The Mighty Thor, God of Thunder, and the cane itself transforms into the hammer Mjolnir.
Likewise, Eric Masterson (AKA 'Thunderstrike') used his cane (AKA the enchanted mace 'Thunderstrike') to transform into his heroic alter ego (...''Thunderstrike').
The H.E.R.O. dials in various incarnations of DC Comics' Dial H for HERO. The twist is the dial's user will turn into a different superhero each time, completely at random. And the dials seem to prefer "unusual" powers to turning the wielder into yet another Flying Brick.
Jack Ryder used the Molecular Transducer to turn into The Creeper.
Rather than transforming him, Rick Jones' Nega-Bands activated a hyperspace link that allowed him to switch places with Captain Marvel, an alien hero who was stranded in the Negative Zone.
The Fly, an Archie Comics hero, originally used a magic ring to transform into his superhuman form; when DC Comics gained licensed use of the Archie superhero characters and rebooted them under the Impact Comics imprint, it was changed to a fly-in-amber amulet.
From Astro City, Winged Victory's logo-shaped necklace serves this function in her civilian form.
Inverted in Wolf, the amulet is supposed to prevent Will (Jack Nicholson) from transforming but at the end he discards it and becomes a full wolf.
The titular mask from the 1994 comedy The Mask (and its comic counterpart).
In Iron Man 2 Tony has a suitcase which turns into and equips him with one of his Iron Man armour variations via something that is pretty much a Transformation Sequence.
In Disney's The Shaggy Dog, Dean Jones gets hold of an ancient talisman with an ominous Latin inscription that turns him into the titular carpeted canine.
Hank McCoy makes a serum that temporarily reverts him from "blue Beast" to "Nicholas Hoult" in X-Men: Days of Future Past. It comes to save him as a Sentinel is about to attack, as the serum not only disguises him but cloaks his X-gene as well.
In Ryuki, though, the cards don't activate transformation: the card holder generates the belt, and transformation is activated when the holder is placed in it.
The Rider Gears in Kamen Rider Faiz fulfill two expectations at once, by using cellphones that slot into their belts.
The Riders in Kamen Rider Hibiki don't use belts but rather soundwave-activated items to transform.
The various Riders in Kamen Rider Kabuto use "Zecters" (robot insects or, in one case, arachnid) that usually mount on the belt buckle; TheBee and Kabutech Riders from The Movie put theirs on wristbands, and Drake and Sasword use their weapons as their trinkets instead.
Den-O, who has a train motif, uses a smart-card akin to the Suica system, with the belt acting as the card reader.
Kiva's Transformation Trinket is Kivat, his kinda-sorta-robot bat sidekick. Kivat's bite causes the belt (and Fangire Game FaceFacial Markings) to appear, and Wataru places Kivat into place as the buckle to initiate transformation. The IXA system requires the IXA Knuckle weapon, which links up with the belt that totally wasn't there before simply being in place when its user needs to trigger a transformation - no special effects, no sound, nothing, just having appeared between shots. The show generally acts as if the Knuckle is the only part that matters, as seen in episodes where you get this sequence: (1) User is hit hard enough to demorph. (2) IXA Knuckle goes flying. (3) Knuckle is picked up and used, activated by linking it with the belt that's now inexplicably around the new user's waist despite not having been there before. (The belt is manually attached in the suit's first few appearances though... but only the first few.)
Kamen Rider Double does things a little differently, as it is not the Drivers (belts) that do the transformations, but "Gaia Memory" modules. A Driver only protects the user from the drug-like effects of the Gaia Memory. The Monsters of the Fortnight uses Memories without Drivers, whereas major characters possess Drivers.
For Double himself, he's made up of two people through Sharing a Body. Shotaro, who forms the body, places the Double Driver buckle on his waist (only the buckle; the strap is generated from it. Several series' belts are this way.) This causes a copy of the whole apparatus to appear on Philip's waist. Both then insert a Memory for element (Philip) and fighting style (Shotaro.) The Memory used by Philip is teleported from his buckle to Shotaro's, who then invokes the transformation. Philip's body passes out as his soul is now within the Double body. When the Fang Memory is used, the suit appears on Philip and it's Shotaro's body that's left uninhabited. When the Xtreme Memory is used, it's a proper Fusion Dance with both bodies and minds merged into Double. Of course, the two-in-one thing creates some problems, since if they lose synch Double becomes paralyzed, and blocking the driver slot can ruin the transformation.
With the belt being manual for Shotaro and just appearing on Philip, both methods of belt summoning are featured in one rider.
Kamen Rider OOO follows in the tradition of Double, with the belt buckle (the OOO Driver) being a holder for the real Transformation Trinket, the Core Medals. Eiji then activates the medals by scanning them with a third device (OOO Scanner) that manifests on the belt whenever he puts on the buckle. As the Rule of Three is a recurring motif, this isn't accidental.
Kamen Rider Fourze returns to the belt buckle (the Fourze Driver) being the real Transformation Trinket rather than the collectible items (the Astro Switches). After putting the buckle on, Gentaro flips the buckle's four front switches (not to be confused with the Astro Switches) on, initiating a 3-2-1 countdown after which he pulls a side lever to transform. The Astro Switches activate different weapons on the arms or legs, and certain special ones enable form changes. However, the Zodiarts use Zodiarts Switches to transform, and the Horoscopes have special unique Switches.
Kamen Rider Wizard has the WizarDriver belt, which unlike most belts is actually worn full-time: most of the time it's in a scaled-back form, having maybe a little larger belt buckle than can be found in real life but it's not too outrageous. Even in this form it can activate most of Wizard's Rings of Power, as all he has to do is hold his ringed hand in front of it. When it's time to fight, then he can turn the thing to the full version that can activate his transformation rings as well. Which of them is the source of his power? Actually, it's the Phantom monster inside him, whose powers he can draw because he was Badass enough to resist the "drive you to despair and burst out of you" process.
Kamen Rider Gaim has each Rider using a Sengoku Driver belt, which transforms them after placing a Lock Seed into the belt's empty slot. The belts initiate the transformation, but the actual power to transform comes from the Seeds. It's a Double situation where the Driver makes a harmful power source safe. Which will come in handy when the Alien Kudzu that spawns the fruits that the Lock Seeds are made from replaces all plant life, as it is expected to in ten years.
Kamen Rider Drive has a fancy belt, and it's called... wait for it... the Drive Driver. However, it's actually a Robot Buddy type AI that is actually actually the inventor of the tech, saved by Brain Uploading. We think. Shinnosuke calls him "Mr. Belt." (Belt-san.) Its ever changing display is full of Mythology Gags, taking on a face like Kivat (the last sentient belt) or spinning lines that resemble the turning of the turbines in the Typhoon belts. But for the first time since Kamen Rider Amazon, the belt isn't the transformation device. The Shift Cars are placed in the arm-worn Shift Brace, for a transformation process that's nigh identical to that seen in ToQger, which it airs alongside. It's shown the belt definitely plays a part, though: in episode 3, he can't transform until Tridoron delivers it Just in Time and he dons it, despite the fact that he'd already been wearing the Shift Brace and had a Shift Car.
Subverted in Kamen Rider The First and The Next, remakes of the original series and Kamen Rider V3 respectively. The "Hoppers" have their traditional Typhoon belts, but no actual transformation is involved - instead, they simply change into their costumes by removing the civilian clothing worn over them and donning their helmets. Sometimes it's even hard to tell when transformation takes place, as we cut from an un-suited Rider spinning the Typhoon's wheel to the suited Rider donning the helmet, with no 'ceremony' whatsoever. The entire suit is like the IXA belt: just in place as needed, no attention called to it, when did that get there?
Interestingly, in some of the older series, posing and saying "henshin" causes the belt to appear in place and activate, without existing beforehand or actually being interacted with in any way. Sometimes it doesn't even appear to activate transformation - it just appears before the rest of the suit out of habit.
A number of modern belts shout out the names of forms and attacks, sometimes with Gratuitous English. Some of the best belong to Faiz (which may have the sexiest voice of any Transformation Trinket), Decade (which has a trademark stutter when announcing finishers—"Final Attack Raido Hi-hi-hi-hibiki", for example), and Wizard (which sings on a loop while primed and waiting to activate a ring).
Then there's the Power Rangers S.P.D. A-Squad. Their unmorphed uniforms are their morphers, apparently: Ranger-colorcoded energy flows from the colored portions of their uniforms to become the Ranger suits.
Kaizoku Sentai Gokaiger follows in the tradition of Kamen Rider Double and Kamen Rider OOO in that the cell phone morphers (the Mobirates) are just holders for the real transformation trinkets, the Ranger Keys: the dispersed essences of every single previous Ranger, materialized in the form of individual Yale keys. Indeed, when past Rangers gets their hands on their respective key, the reabsorbing of its power reverts it back into their own morpher.
Power Rangers Turbo also had the keys be the real power source: the keys were put in a number of devices, and got all glowy to make other things happen (in the manner of several mystical power sources that reserve the right to occasionally do more than what they're normally used for.) However, it's easy to forget because they do this much less after The Movie.
Before them, Mighty Morphin' Power Rangers' Power Coins did way more than morph the Rangers (which, under special circumstances, they could do without the morphers.)
In Power Rangers RPM, it's Engine Cells. A variety of them are placed in a variety of things, including the morphers (cell phones for the primary colors, wrist-worn for Green and Black, weapons for Gold and Silver) and the Zords (to make them grow, because they're pocket-sized by default). However, they're much less of a plot point unto themselves than Ranger Keys or Turbo Keys, let alone Power Coins or Core Medals.
In Engine Sentai Go-onger, which is a more lighthearted series, the Engine Souls are more plot points. An Engine is an alien Robot Buddy that can only operate for so long at its proper Humongous Mecha size under Earth's conditions, and after a time will revert to Sleep Mode Size, which takes the form of an Engine Cast (the toy-sized body) and an Engine Soul (exactlywhat the name implies.) When an Engine Soul is placed in a weapon, that allows the Engine to operate it from within. When placed in the morpher, the morpher creates a little cartoony hologram of the Engine. The Change Souls that power the morphers for transformation, however, aren't the soul of anything, and only do that.
Zyuden Sentai Kyoryuger, being a dinosaur series, decides that the most logical choice for the transformation trinkets is a large gun in which a battery is inserted. The guns, however, are more like the aforementioned belts belonging to Kamen Riders Double, OOO, and Wizard; the batteries, charged up with the user's bravery, are what cause the transformation. The exceptions are the Sixth Ranger, who was born before the existence of guns and uses a large wrist-worn crossbow-like device, and the first two Guest Star Party Members, who are already dead and now manifest as spirits that can channel their bravery directly into their batteries.
Kyoryuger's successor To Qger returns to the wrist devices for the main five. However, they're two-piece: a model of each Ranger's personal train car mecha is inserted into the To Q Changer. Yes, they are interchangeable. Each train comes with a model to be used in the Changer to summon it, or in some cases use its power at small size. They're similar to the Beast Batteries of Kyoryuger.
The Ultra Series typically has the human host of each title character use an activator device to initiate the change.
For the first one, Ultraman, it was called a "Beta Capsule".
In Superhuman Samurai Syber-Squad, the procedure was more involved than most cases: Sam had to bring up the Servo program on his computer, and then play a certain chord on his guitar. The process was automated in a wrist-worn device, so he can just push a button if he wants, but only seems to do this when he can't do the normal procedure from home.
For the others, each says a code and does a pose. For Tanker, he tends to clap his drumsticks together, giving a jolt of electricity nobody but the viewer seems to see, when saying his phrase. This seems to be Rule of Cool or Rule of Funny, because there is really no reason a totally-non-electronic pair of wooden drumsticks would (or could) have anything to do with the program.
In the Stargate SG-1 episode "Foothold", the aliens use a device to appear as members of the SGC. Later in the episode, Sam Carter gets ahold of one of these devices and uses it to turn into her teammate Daniel Jackson.
The Makai Knights of Garo transform by using their weapons to draw circles in the air.
Tomica Hero Rescue Fire combines this with Calling Your Attacks the device in this case is a megaphone they have to yell "Chakusou!" into. Fire-4 and Fire-5 use their weapons (swords with microphpones in the hilt) to transform.
Merlin had two villains with these in the episode "Gwaine". They used them to take the identities of knights they killed. They were crystals of some type, activated with a bit of the victim's blood.
Similar to the Wolf one above; Red Riding Hood's signature red cape is what keeps her genetic werewolf form condition at bay every full moon. She often forgoes it when she needs to transform as a distraction, however.
Regina gives mermaid Ariel a bracelet that allows her to assume human form.
BIMA Satria Garuda has the red Power Stone for the hero, while apparently differently colored ones are used by villains.
The Manaketes in the Fire Emblem series generally require an item called a dragonstone to transform into their true forms. As dragons, they generally have little trouble destroying the opposition, but once the stone runs out of uses and breaks, they're helpless unless you can somehow manage to find a new one. Story wise, Bantu, Fa, and Myrrh all get their stones stolen from them; the former simply requesting that you catch the thief responsible so he can help you (This happens on two occasions in two different games, no less), and the latter two needing to be rescued from the enemy.
Remedied in Fire Emblem Awakening, where dragonstones are apparently common enough to be purchased from shops, despite the fact that manaketes have become a race of the legends. The same game introduces the Taguel race, who require beaststones to transform (Which likewise, can be purchased from shops). Both races can simply opt to rely on the strength of their human forms by taking up a normal class, as well.
In The Legend of Zelda: Twilight Princess, Zant curses Link into permanent Wolf form using a black crystal. After the curse is broken, Midna is able to use it to transform Link at any given time. Also, the first set of Plot Coupons essentially act like this for Midna.
Kingdom Hearts outright states that Sora's newer, black clothes are the Transformation Trinket for his Drive-Forms and the only thing that actually changes when he "drives". (Except for Anti-Form, where his whole appearance changes to that of a Heartless) They also help him absorb MP from the air, which is why the Magic-system in Kingdom Hearts II differs from that of the first part.
More from Kingdom Hearts, in Birth by Sleep the pieces of armor that Terra, Aqua and Ven wear in their traveling clothes are used to activate the transformation into their full armor.
The biometals in Mega Man ZX and Geo's Transer/Star Carrier (With Omega-xis) in Mega Man Star Force. Most morphing adversaries in Star Force 2 use Ancient Star Carriers, which are exactly the same only built directly with Murian technology.
The anime version of Mega Man Battle Network had Synchro Chips combined with the already existing PETs. The transformation only works in areas that has enabled Net Navis to manifest in the real world.
Seems in every Mario Game, there's a new item that gives Mario and Luigi a new form in addition to new abilities.
Super Mario Bros. 3 introduced the Super Leaf that turned Mario into Raccoon Mario (raccoon ears and a tail, limited flying/gliding abilities). Later in the game there's the Frog Suit (precision swimming), Tanooki Suit (Raccoon abilites plus transformation into a Tanooki statue that can crush virtually any enemy), and even the rare Hammer Suit (throw incredibly lethal hammers, the shell itself is impervious to fireballs). The GBA version of this added a "Feather" granting Cape Mario, like in Super Mario World.
Super Mario World retained the original three powerups, replacing the Leaf with the Feather for Cape Mario. A rare balloon powerup inflates Mario, allowing him to float in the air for a limited time.
Super Mario Land was like a handheld expy of the NES classic, with the orignal power ups, only the Fire Flower is replace by a flower that turns Mario to Superball Mario.
Super Mario Land 2: 6 Golden Coins continues the tradition with the original three power-ups and a new flying ability, this time a carrot that gives Mario bunny ears that make him fly for a bit. Also, because of the graphics for the black-white gameboy, Fire Mario had a feather on his head here. Also during the final battle with Wario, the villain goes through the three power ups (no star) in the fight.
Super Mario 64: Ditch all the powerups for three hats: The blue cap make Mario invisible (and thus, invulnerable) and able to pass through certain walls; the Metal Cap turns Mario into an invincible heap of walking metal (complete with a remix of the Super Star music), and the red Wing Cap allows limited flight. Super Mario 64 DS however made invisibility exclusive to Luigi, metal exclusive to Wario, Yoshi gets to spit fireballs, and Mario can blow-up like in Super Mario World or gets a feather and become Wing Mario.
Super Mario Sunshine didn't have Mario change forms, but his talking-weapon of choice, FLUDD can switch modes with the right power up, to have Mario hover, jet up in the sky or to run with Super Speed.
New Super Mario Bros. retains the Super Mushroom and the Fire Flower. There was also a temporary transformation with a Mega Mushroom, turning the user (even a boss used one!) into a giant unstoppable machine! There's also a new power-up, a blue shell that turns Mario or Luigi into Shell Mario (slide along the ground just like a turtle shell, knocking out everything in your path). There's also the Tiny Mushroom, turning Mario into a tinyOne-Hit-Point Wonder that can float through the air, fit into incredibly small pipes, and and run across water.
There are seven forms this time in Super Mario Galaxy (it even brought the Fire Flower to 3-D, if only temporarily). There's also an opposite called the Ice Flower that grants Ice Powers. The Super Star was replaced by a Rainbow Star wih an added speed boost, and this game has two flying abilities: a Red Star (exclusive to the Hub Level, sorry) turns Mario into Flying Mario, and a Bee Mushroom that gives him the hovering Bee Suit. There's even a Boo Mushroom that turns him into, (yep, you guessed it) Boo Mario who can float in the air and pass through certain walls. And finally there's the Spring Mushroom which turns Mario into Spring Mario and grants him super-jumps.
New Super Mario Bros. Wii has an Ice Flower which grants different ice powers from the Galaxy version, a Propellor Mushroom for the new flying form (a jumpsuit with a propellor on the top), and a Penguin Suit (ice powers, slide into enemies, and improved swimming). There's also the Mini Mushroom from the DS version and the usual Super Mushroom, Fire Flower and Super Star.
Super Mario Galaxy 2 continues the tradition, bringing over almost all of the powerups from the first game, and introducing some new powerups: the Drill, which lets Mario burrow through dirt, the Cloud Flower, which allows Mario to create his own cloud platforms, and the Rock Mushroom, which turns Mario into a rolling rock to plow through enemies and smash into things. Yoshi also has his own powerups: a Dash Pepper that makes him run really fast for a time, a Blimp Fruit which allows Yoshi to float into the air for some time, and a Bulb Berry that lets Yoshi unveil hidden platforms (similar to the Matter Splatter levels of the first game, only Yoshi provides his own spotlight).
Super Mario 3D Land saw the return of Tanooki Mario minus the flight and statue capabilities (in the normal world version anyway), the power-up based hit point system (thankfully Mario's default form is super this time) and the first ever appearance of an unlimited fire flower in a 3D Mario game. It also introduced the boomerang flower, which allows Mario to throw boomerangs.
New Super Mario Bros. 2 also brings back the Super Leaf from Super Mario Bros 3, this time granting the flight capabilities(and the return of the P-meter). There's even a Invincibility Leaf that turns Mario into the near invincible White Raccoon Mario if the player loses 5 lives in one world.
Speaking of new, the Gold Flower is introduced in this game, granting Mario and Luigi a more powerful fireball attack that creates shockwaves and can take out many enemies, giving the player more coins. This power lasts till you complete the level or get hit (demoting you to Fire Mario/Luigi in both cases).
New Super Mario Bros. U sees the return of the Super Mushroom, Fire Flower, Ice Flower, Mini Mushroom, Propeller Mushroom, and Penguin Suit. It also sees the introduction of the Super Acorn (which turns the recipient into a Flying Squirrel form, effectively a modified Raccoon Suit that looks like a circus costume) and the P-Acorn (which turns the recipient into P-<recipient>, a powered-up Flying Squirrel form capable of infinite flight).
Super Mario 3D World sees the return of the 3D Land-style Tanooki form. It also is shown to feature a Cat Bell that gives the recipient a Cat Suit (no, not that kind) that allows them to run up walls and tackle enemies, among other things.
Pokémon Platinum has two items for the purposes of transforming two Pokémon between their alternate forms. The Platinum Orb allows Giratina take on its Origin Forme in the real world, and the Gracidea flower allows Shaymin to take on its Sky Forme. For the latter though, the transformation does not work at night and it can be canceled by Shaymin being frozen.
Pokémon X and Y introduces "Mega Evolutions", which allows a small selection of pokémon to take on their Super Mode forms during battle. The Transformation Trinket required to make it work comes in two parts: a Mega Stone to be held by the pokémon, with every pokémon that is able to mega evolve needing a specific stone, and a Key Stone to be held by the trainer (usually in the form of an accessory such as a ring or braclet), which is used to actually trigger the evolution.
In addition to the usual Mega Manning, some Kirby games have Copy Essences lying around that grant an ability when touched. In Milky Way Wishes, this is the only way to use special abilities (they're permanent and can be switched at will though).
Later games give Kirby a souped-up special ability, usually in the form of a wand or sword. The item bounces around the screen in place of an Ability Star when you let go of it, implying that it's the source of the ability instead of Kirby's copying power.
In Muppet Monster Adventure , the main character Robin the Frog can channel the powers of the monsters that his uncle Kermit and his friends have been transformed into with the help of special amulets. The Werebear amulet turns him into a werebear with the power to climb any climbable surface, the Muck Monster one turns him into a swamp creature with the power to swim underwater, the Nose-feratu one gives him flight and turns him into a bat, the KerMonster one turns him into a frankenstein-like creature with super strength, and the Bride of KerMonster one turns him into a Bride Of Frankenstein-like character with a superpowerful karate chop that can smash down doors.
Silent Hill 3 has the Princess Heart Transform Costume, which is activated by a key-shaped trinket.
The accessories in Princess Debut let your main character don one of 20 different ballroom-ready costumes.
In Long Live the Queen you will need to go through quests to obtain your mother's magic crystal in order to officially become a magical girl and get the sparkly transformation sequence. After doing it once, though, the crystal becomes part of you, so you can't lose it.
Myan's collar in Cat Nine allows her to transform into a cat girl or various animals. Removing it turns her back to normal and removes other powers (like human speech) in the process.
In Sluggy Freelance, Zoe's transformations into a camel are tied to a necklace given to her by Torg. She wishes she could conveniently lose the necklace somewhere, but after her first transformation it changed into a seemingly unremovable tattoo. Zoe doesn't mind the tattoo itself too much, but desperately wants to get rid of it so her friends can't turn her into a camel whenever they're feeling mischievious.
It's not so much a brooch as a magical jewel embedded in her chest. Which does deal with the whole "broken, lost or stolen" thing - if any of those happened, Yuuki would be much more worried about bleeding to death than not having access to Valkyrie powers.
Each member of the Panthera team has an amulet on a necklace "that contain[s] minerals that ease the transformation and help focus [their] elemental power" .
In El Goonish Shive, the Transformation Gun derived Cat Belt which has two buttons "Feline" and "Human".
Darkbolt has the Demon Orbs which transforms the 'soul merged' humans into their demon armors. In one case the Phoenix Samurai changes due to an angel.
In To Prevent World Peace, every Magical Girl has one. Focus items can be summoned at will (sorta like Sora's keyblade) and are necessary for transforming. If they ever break . . . it's not good.
Homestuck has the rings and scepters of the Prospitian and Dersite monarchs, which apply the powers granted by the players' prototypings to the user. They don't work for humans though.
Magical 12th Graders: The pin, a magical artifact that can transform anyone into a magical girl (including boys, though their outfits are suitably masculine). The cubics, little pieces of the pin that broke off, can be used for the same purpose, though they're less powerful.
Was the Holy Grail a 7-11 Big Gulp cup that someone spiffied up? The magic of it all was very dazzling, but I should have KNOWN something was wrong from the start. This wasn't how it happened on TV. It wasn't this much of a hack job.
The Dimensional Guardians from the web fiction serial Dimension Heroes use bands called Guardian Bands to transform into their battle armor. Before the protagonists discovered their powers, they were often thought of as pieces of jewelry.
Oingo uses a belt buckle for his disguises in Vaguely Recalling JoJo, but it still has the flaw of giving him a shirt labelled Oingo.
In the Ace Ventura episode "Howl of The Weremoose", Drew Talbit uses a moose talisman to change into his weremoose form. It is also revealed that smashing the talisman will break the curse and turn all the weremoose ghouls back to normal. Unfortunately, Ace doesn't read the fine print which says he who destroys the tailsman gets the antlers of the grand high moose himself.
Ben 10: Ben wears the Omnitrix on his wrist, and frequently calls it a watch (even though it isn't).
Subverted by Ben not originally knowing the device's true title, and the Nickname sticking.
As the series went on he started calling it by the Omnitrix and stopped calling it "The Watch" altogether.
One episode of Danny Phantom involved an amulet necklace that would turn the wearer into a dragon whenever they get angry.
El Tigre: The Adventures of Manny Rivera: a spinning belt buckle.
The Skysurfers from Skysurfer Strike Force use watches called Digitrans to transform themselves into their hero identities and their cars into flying surfboards.
1950s cartoon Tom Terrific was able to morph into anything thanks to his magic funnel.
W.I.T.C.H. had the Heart of Kandrakar/Candracar. The funny thing about it is that it had to be used for all five girls to transform and if one of them wasn't around when Will used it, she had to transform them then.
Lance and Ilana have watches that allow them to put on/transform into their Manus and Corus armor, respectively. Octus, already being a robot, doesn't need one of these, as he's able to merge all three of them into the Sym-Bionic Titan.
In order for the titular hero of Dragon Booster to transform, he needs an Amulet (which holds a "key" of sorts) and Gauntlet. If he loses either one, he is stuck in his civilian form. It also requires that he shout a Catchphrase, but that's another trope entirely.