By swiping a Rider Pass across the SetTouch, Kamen Rider Den-O is able to pass through four forms, each with their own strengths and weaknesses. Each of the forms (excluding Plat Form, Climax Form, Liner Form and Super Climax Form) is based on a specific mythological hero along with motifs relating to the respective legends. However, when any of the Taros uses the Rider Pass, they bypass the Plat Form to assume their respective Den-O Form. Its overall motif is the shinkansen. The Den-O Belt works by drawing in free energy from the surrounding area, allowing it to generate the Den-O suit, which consists of a soft but durable material called Aura Skin and tough armor called Aura Armor. As the name implies, Aura Skin and Armor are attuned to the wearer's individual aura; because of Ryotaro's weak will, he can only generate the basic Plat Form. However, possession by Imagin causes the Aura Skin and Armor to reconfigure themselves into a form that matches the Imagin's will, as well as generating a face mask called a Denkamen.
Plat Form
This is Den-O's most basic form and it is a black and white form that is activated when first initiating a transformation using a Rider Pass. If the corresponding color-coded button is selected, then by swiping the Pass once more, a form change will occur. Ryotaro rarely uses this form, as it does not have sufficient armor and pales in comparison to the abilities of the other forms. Only when he does not want the assistance of the Taros or is transitioning into Liner Form. Plat Form is designed and named after a railway platform.
Sword Form
Sword Form is Kamen Rider Den-O's first form, used while Ryotaro is being possessed by Momotaros. When in Sword Form, he uses the DenGasher as a standard sword-style weapon. Sword Form is the most balanced of all his forms and most used, possibly because of Momotaros' prominence among the other Imagin. Momotaros himself once assumed Sword Form during the events of "Final Countdown" to battle Kamen Rider Yuuki. Every so often, Momotaros will develop a new Finishing Attack that he uses to finish off Imagin, and has developed Parts 1, 2, 2' (2-dash), 3, 5, Special Version, and the other-Imagin-assisted Final Version, although parts 2, 3, and 5 are essentially identical. Special Version is nearly identical to 2, 3, and 5, but uses a double Full Charge and only strikes the enemy on the rebound, while Final Version is an extended attack that uses the energy of the other Taros, Sieg, and Zeronos. Sword Form also has a triple roundhouse Rider Kick that Momotaros refers to as his "Super Finishing Attack", though this is never used during the series itself but within the Cho Kamen Rider Den-O & Decade Neo Generations: The Onigashima Warship movie. The eyes on the Sword Form's Denkamen resemble two halves of a peach, the fruit associated with the story of Momotaro, the Peach Boy, which it is based on. When possessed by Momotaros, Ryotaro assumes the mannerisms of a self-absorbed punk. His hair has a single red streak and he gains more muscular definition and the physical strength to go with it. He takes on an extremely rash and ignorant personality, just like Momotaros.
Rod Form
Rod Form is Kamen Rider Den-O's second form, used while Ryotaro is possessed by Urataros. When in Rod Form, the DenGasher is able to transform into a rod-style weapon. Rod Form gains the ability to perform underwater, extra protection, and increased kicking power but suffers a loss of physical powers and jumping height. Urataros himself assumes this form when he borrowed Ryotaro's Belt to fight off the Mole Imagin he tricked, and again during "Final Countdown" to fight the army of revived Imagin. This form is one of the least used of Den-O's forms, as Urataros rarely fights. However, when the fight occurs in the proximity of water or if the other Imagin are unable to fight, Urataros will assist with his fishing skills. Rod Form is also only one of two forms that uses a Rider Kick as part of its standard finisher, pinning the enemy with the rod as a harpoon before executing the flying side kick. The Aura Armor and Denkamen resembles a sea turtle, the animal associated with the legend of Urashima Taro, upon which Rod Form is based. Ryotaro assumes the mannerisms of a teen idol casanova, with the abilities to woo women and convince almost anybody. His hair is straight and has a single blue streak going through it. He also wears a pair of black rimmed eyeglasses. He takes on a very professional and crafty personality, exactly like Urataros.
Ax Form
Ax Form is Kamen Rider Den-O's third form, used while Ryotaro is possessed by Kintaros. When in Ax Form, the DenGasher is able to transform into an axe for short ranged attacks. Ax Form gains increased punching power and defense but suffers a loss of speed and kicking power. This form is the physically strongest of the Den-O forms, due to Kintaros's own brute strength. Kintaros himself assumes this form using the Den-O Belt to help Ryotaro fend off an army of Imagin in the past, and again during "Final Countdown" to fight off the army of revived Imagin. Kintaros tends to fight as Ax Form whenever he senses crying or he hears someone near Ryotaro say anything resembling the Japanese word for "cry". Ax Form's Aura Armor is golden yellow and the motif is that of an ax, both symbols prevalent in the story of Kintaro, the Golden Boy, upon which Ax Form is based. The Denkamen is also shaped like the Kanji for the word "gold.” When possessed by Kintaros, Ryotaro assumes the mannerisms of a martial artist. His hair grows such that he has a ponytail with a golden yellow streak through it. He takes on a very noble and firm personality, exactly like Kintaros. However, his increased strength often results in self-injury and clumsiness. His usual attire in this state is a yellow-and-black striped kimono.
Gun Form
Gun Form is Kamen Rider Den-O's fourth form, used while Ryotaro is possessed by Ryutaros. When in Gun Form, the DenGasher is able to transform into a gun for short range and long range attacks. Gun Form displays an increase in speed, power, and defense, easily making it the most superior of the four basic forms. Ryutaros tends to fight as Gun Form at his whim, as he is able to overpower the other three Taros and sometimes Ryotaro himself. However, he also tends to cause more collateral damage than the other forms because of his low precision and the high power of his ammunition. Ryutaros himself once assumed Gun Form during the events of "Final Countdown" in order to battle the revived army of Imagin. Ryutaros incorporates break-dancing moves into his fighting style in Gun Form, making his movements somewhat unpredictable. The motif of Gun Form is that of a dragon, the creature associated with the story of Tatsu no Ko Taro, Taro the Dragon Boy, upon which Gun Form is based. When possessed by Ryutaros, Ryotaro assumes the mannerisms of a 80s hip-hop break dancer. He grows one long purple and black streak of hair and is always seen wearing a brown cap. His reckless personality infests itself upon Ryotaro, and usually causes him to destroy whatever is around if he gets out of control. One such incident occurs during the events of Climax Deka when Ryutaros in Ryotaro's body arrests everyone who he thinks looks suspicious and therefore must be part of the "Evil Organization" advertised by Negataros.
Wing Form
Wing Form is Den-O's fifth form, used while Ryotaro is possessed by Sieg, first used to protect Shiori Takayama from the Scorpion Imagin, though it is unable to finish the fight due to Sieg's dwindling strength. Wing Form is used again during the trip to the Edo period to fight the Cobra Imagin. Sieg himself once assumed Wing Form during the events of "Final Countdown" to assist in fighting Shiro's army of revived Imagin. Den-O Wing Form uses the DenGasher as two separate weapons, a boomerang (Sword and Grip) and a handaxe (Ax and Gun) when in Wing Form. Though resembling Sword Form, the Aura Skin becomes gold, different shoulders are used, and the Denkamen's visor is, blue swan-like wings, associated with the bird of the story of Swan Lake, upon which Wing Form is based. In order to access Wing Form, the golden Wing Buckle must be placed over the Terminal Buckle. This form gives Den-O an increase in mobility even greater than what Gun Form could provide. When possessed by Sieg, though they never forged a contract, Ryotaro assumes the mannerisms of a prince. His hair is styled in cornrows and has several white highlights with one longer white and black lock on the left side of his face, and a white feather boa appears around his neck.
Climax Form
Climax Form is Den-O's sixth and strongest form in the series. It is accessed after all of the Taros except Momotaros fade away and Ryotaro's desire not to lose them creates the K-Taros which restores the three Imagins' bond to Ryotaro. After entering the code 3-6-9-# and attaching it to the Den-O Belt, Den-O becomes Climax Form. Climax Form is basically a stronger version of Sword Form that incorporates the Denkamen of the other three primary Den-O forms onto various parts of the armor. Due to the nature of Climax Form, the Taros need to work as a unit to fight, and their strengths are incorporated into Climax Form. Rod Form's mask is on the right shoulder with Urataros controlling the right arm, Ax Form's mask on the left shoulder with Kintaros controlling the left arm, and Gun Form's mask on the chest with Ryutaros control the legs. These three masks can travel along tracks on Climax Form's body to align on the leg or arm to enter Kick Mode or Punch Mode, with the three Taros controlling that one limb while leaving Momotaros to control the rest of the body to perform "Charge and Up" attacks.
Liner Form
Liner Form is Den-O's final form in the series, gained when the Taros created the DenKamen Sword for Ryotaro to use when their synch with him failed. In order to transform into Liner Form, the Rider Pass must be put in a slot in the back of the blade of the DenKamen Sword. This form, solely controlled by Ryotaro with the Imagin using the DenKamen Sword as a means of communication. Ryotaro also utilizes each Imagin's respective fighting style when their mask is selected on the DenKamen Sword. It soon becomes the only form Ryotaro is able to use while fighting in the past after all of his other forms become inaccessible. Although considerably versatile, Liner Form does not hold any significant strengths over Den-O's other forms aside from having the fastest running speed, which sometimes leaves Ryotaro at a disadvantage. By the time the events of Saraba Kamen Rider Den-O: Final Countdown take place, Liner Form is Ryotaro's default form, accessed through the Rider Pass, the K-Taros, and the Den-O Belt.
Super Climax Form
Super Climax Form is Den-O's eighth, strongest, and true final form, exclusive to the Cho-Den-O Series and first appearing in Cho Kamen Rider Den-O & Decade Neo Generations: The Onigashima Warship. It is identical to the original Climax Form in being a stronger version of Sword Form that incorporates the DenKamen of the other three primary Den-O forms onto various parts of the armor. However, Super Climax Form also includes Wing Form as wings on the back of Den-O and a Wing Form insignia on the mask. It is also physically stronger than Climax Form and has the ability to use the wings to fly up and execute a stronger version of Climax Form's Rider Kick.
Altair Form
Altair Form is the default form of Zeronos, using the Zeronos Cards to evoke the change. It is named after the star Altair in the constellation Aquila the Eagle. The Denkamen is made up of two green bull's head-shaped visors, the same motif as ZeroLiner Drill, which is based on the mythological occupation of Altair as a cowherd. These form from two bull's head-shaped train cars that ride along the tracks on Zeronos' helmet. The rest of the body has a set of converging train tracks that are almost like a zipper. Altair Form has generally balanced abilities comparable to Den-O's Sword Form. When Yuto becomes Altair Form, he says, "I'll tell you this for starters…" followed by something that concerns Yuto's strength, usually with "I am fairly strong," or how annoyed he is. Another common trait is occasionally when the transformation is completed, Zeronos will summon a lightning bolt down from the sky to strike in front of him, or lightning will simply flash in the sky above. After using the Red Zeronos card, Yuto's own memories were used to cause Altair Form to "rust", changing it into Zero Form, although the future Yuto Sakurai is still able to assume Altair Form. In the final fight, Yuto once again uses this form using the last of original Zeronos cards. Later during the investigation of Negataros, Yuto returns with a new set of Zeronos cards, able to assume Altair Form once again.
Vega Form
Vega Form is the form of Zeronos that comes from being possessed by the Imagin Deneb. It is named after the star Vega in the constellation Lyra the Lyre. It has a series of cannons on its shoulders called the Zeronos Nova. In the process of transforming into Vega Form, Deneb's body makes up various aspects of the form: his face is on Vega Form's chest, his hands form the Zeronos Nova, and his cloak becomes the cape. The Denkamen is made up of a spool of thread, as Vega is a weaver, that opens up into a star shape. Vega Form is physically stronger than Altair Form, although suffers a loss of speed and overall agility. Whenever Deneb possesses Yuto to assume Vega Form, he says, "I'll tell you this for starters…" following with an honest fact that annoys both the opponent and Yuto. Vega Form is accessed for the last time during the final battle, playing a part in the destruction of the Death Imagin, although Yuto assumes the form once more in the fight against Negataros.
Zero Form
Zero Form is a form of Zeronos that Yuto can assume, visually similar to Altair Form, but with a red color scheme and golden trimmings. After Yuto used the first Red Zeronos card, it used his own memories as a catalyst to cause the green portions of Altair Form to rust dramatically, turning them red and giving the armor a rustic-like texture while increasing its capabilities. As he did with Altair Form, Yuto can assume Zero Form on his own, with access to the Denebick Buster. Zero Form is lost when Yuto is defeated by the Death Imagin using his last Red Zeronos card, but is later regained through unknown means in time to aid the DenLiner crew against Shiro and the revived Imagin. Possibly because of memories of him are still being made in the present. For some reason, instead of announcing the form's name, the transformation is accompanied by "Charge and Up".
Series:  Kamen Rider Den-O