Zack

"I am Team Baron's leader Zack! Armored Rider Knuckle!"

- Zack's roll call upon transforming for the first time

Zack (ザック) is a former member of Team Severe Beat and the current leader of Team Baron. As the group's former second-in-command, he was promoted to the position when their leader, Kaito Kumon, stepped down to focus on the mystery surrounding Helheim Forest. In his first Inves Battle, he is shown using the Matsubokkuri Lockseed. He later receives a mass production Sengoku Driver from Kaito who obtained it during his capture and escape from Yggdrasill. With the Sengoku Driver he received and his own Kurumi Lockseed, he is able to transform into Kamen Rider Knuckle (仮面ライダーナックル), the second rider in Team Baron.

History
Before the Inves Games, Zack was part of a Beat Rider team called Severe Beat. His team lost to Team Baron, which prompted Zack to join them. Zack became one of Team Baron's top dancers, and one of Kaito's most devoted followers, developing a cocky attitude in the process towards other Beat Rider teams.

Zack quickly befriended fellow Team Baron dancer Peko, and the two formed a habit of helping Team Baron to cheat in the Inves games, typically by using a slingshot to cause their enemies to drop their Lockseeds and lose control over their Inves, thereby losing the battle. While this strategy initially worked, Team Baron's leader, Kaito Kumon, was furious upon discovering their cheating ways, declaring it humiliating towards him as he destroyed the slingshot. Despite being discovered, Zack and Peko remained with Team Baron. As Team Gaim tries to convince the Beat Riders to participate in their dance event, in order to help convince Zawame City they were not responsible for the Inves invasion, most of them denied the offer thanks to Kaito's disinterest. After they left, Peko reveals his purpose of joining Team Baron, that he had a genuine love of dancing, which prompts Kaito to quit the team, relinquishing leadership to Zack as well as giving him the Sengoku Driver he had stolen from Ryoma Sengoku. Later, Team Baron went to Team Gaim's garage to participate the event, also telling them that Kaito quit their team. Before the event could start, Oren arrives to crash the event. While Gaim fights Bravo and the Inves, Mitsuzane is about to transform, but Zack stops him as he tells him that the dance is also theirs. Using the Kurumi Lockseed, Zack transforms into Armored Rider Knuckle and assists Gaim. While fighting, the rest of the remaining Beat Riders joins in the event as they are about to dance. Despite the interference of Oren and Hideyasu Jonouchi, the event is a success, and the blame lifted off the Beat Rider's shoulders.

Later, Knuckle fought the two Inves in Zawame. After defeating them, Zack and Peko notice the injured citizens have Helheim's plants growing from their wounds, making the citizens panic and forcing them to back off. Oren arrives along with Hideyasu and still mocks the Beat Riders until three Inves appears.

Later, he and his team were on a dancing join, they saw "Kaito" (who is actually Shapool), and offers him back to their team, with Shapool agree. However, Zack and his team finds "Kaito" weird and changed into happy man all the sudden, something Kouta and Mai doesn't expect to see "Kaito" like that, as Kouta warned Zack not to trust "Kaito" yet, if turns out to be an impostor. When Shapool's identity is exposed when he uses Kaito's Sengoku Driver and suddenly attacked by Oren out of fear, Zack fights Oren in Shapool's place, leading Mai and Shapool to be able to escape, in same time the real Kaito found Shapool and drag him into abandoned place, revealing a true agenda of Shapool's butler, Alfred, to kill him and take over his family fortune for his greediness. With Kaito and Shapool manage to escape Alfred, due to a prototype Dragon Fruit Energy Lockseed's effect he use as a New Generation Rider Tyrant, Zack and his team are attacked by Alfred, leaving Peko to send Kaito a message to bring Shapool, while using Zack as a hostage to make sure if Kaito didn't bring Shapool, Zack will be killed. Once Kaito arrived rescueing Zack in an abandoned factory, with Shapool is save with Peko, hiding from Alfred, Zack takes a hiding place to cover from battle to heal his injuries from Tyrant, until Baron defeats Tyrant and being aided by Peko to the hospital, same time Shapool finally goes home at peace and thank Kaito for his inspiration to trained himself to be become stronger.

Mai later informs the Beat Riders about Yggdrasill's plan as she proposes the Beat Riders reveal it to the public However, a horde of Inves appears, forcing Kouta, Zack and Mitsuzane to fight them. Unknown to him and his friends, it was Mitsuzane's doing on protecting Yggdrasill's secrets from bankruptcy, and framed Kouta to think he "hurt" Mai's feelings.

Zack joins Kaito to fight Demushu, however the Over Lord escaped from them. Later, they witness the Kurokage Troopers guarding Zawame from Inves. While wondering in Zawame, they were shocked to see Yggdrasill Tower being covered by Helheim's plants as Inves started to appear from the tower. They then join Marika to fight an army of Inves as Hideyasu and Oren join them as well. After the Inves were defeated, Demushu appears and overpowers the Riders with his new evolved form. Struggle to fight the Over Lord, Kouta appears as he defeats Demushu with his new Kiwami Arms.

Before Mitsuzane's arrival and Kaito's left to a still opened Druper with Kouta, Zack was given a Suika Lockseed by Kaito, which is later used by Gridon. Fed up with Redyue's announcement on her species' invasion and going to used his people for a sacrifice, Zack goes to the Over Lord with Jonouchi and Oren accompanies him until they find and rescue civilians attacked and caught by an Inves army. When Kouta discovers Mitsuzane's true colors to the Beat Riders, Zack becomes enraged to hear, outraged he had been tricked the whole time.

Zack later aids Kouta in fighting Duke, but was easily defeated by the New Generation Rider. With the interference of Baron, Ryoma informs the others of his plans. Zack and the others does not appear to trust Ryoma, further exacerbated by Yoko's distrust of him. Despite this, left with little choice Zack joins in with the other Armored Riders and follow Ryoma's plan. Receiving the Suika Lockseed from Kaito, he changes into Suika Arms and along with Baron and Marika, they successfully hold of the Suika Arms and Tulip Hoppers for Kouta and Ryoma to enter the enemy's base. After Kaito destroys the machine that Redyue and Mitsuzane stole the life force from each people, Zack and Yoko evacuate everyone out of the room while Kaito searches for Kouta.

After discovering Kaito's transformation into Lord Baron, Zack faces a conflict of loyalties, unable to decide if he should help his friends or aid his once-leader in conquest. In the heat of battle, Zack seemingly makes his decision to aid Kaito by helping Yoko dispose of Gridon, destroying Jonouchi's Lockseed and Sengoku Driver in the process. After Kouta returns to battle Kaito, Zack interrupts their battle, transforming into Knuckle to battle Kouta in his Kiwami Arms form. During the battle, Zack discretely tells Kouta to retreat for the time being, revealing his true loyalties and ulterior motives.

Later, Zack passes along a note to Peko, telling him to deliver it to Oren. At Drupers, Peko delivers Zack's message to Oren, who creates a miniature bomb at his request, which he later picks up while telling them he will be the one to stop Kaito. Sometime later, Zack meets with Yoko and Kaito atop a building in the abandoned Zawame, with Zack secretly planting the bomb while walking away holding the detonator. Unfortunately, Yoko notices Zack's suspicious movement, and acts just in time to move Kaito out of the bomb's blast radius, taking the brunt of the explosion herself and getting tossed to the ground below. Back atop the building, Kaito and Zack transform to battle, resulting in Zack being brutally defeated, his Driver and Lockseed damaged beyond further use. Zack defiantly tells Kaito that the others will still come after him, with Kaito acknowledging him before he faints.

3 months later since the disappearance of Helheim, Zack watches happily as the united Beat Rider teams dance to an audience of the reclaimed Zawame City, standing with the help of a crutch. He catches the still-despondant Mitsuzane watching from afar, and begs the former Gaim member to dance with them again, although Mitsuzane declines. Then after his legs are healed for 4 months, Zack and his Beat Riders team sees Mitsuzane again and try to convince him to join with them again but he refuses. He helps Takatora, Oren and Jonouchi in fighting a Grasshopper Monster that Kougane as Kamen Rider Jam summoned, but they are powerless to defeat the monster as they are no longer Armored Riders. After Kougane's defeat, Zack and the rest of his fellow former Armored Riders reunite with Mitsuzane.

Kamen Rider Gaim Gaiden: Kamen Rider Knuckle
One year after Kaito disappeared, Zack decides to travel to New York and leaves Team Baron's leadership to Peko. However upon arriving in New York, he is confronted by Peko's older sister, who tells him that Peko is in deep trouble. Apparently Shura, a former member of Team Baron that Kaito previously exiled, has become a thug, forming a gang called Neo Baron. Zack returned to Zawame City to set things right.

Personality
Zack serves as second-in-command in Kaito's absence. He is an excellent dancer and loyal to Kaito when his leader was human. While in an Inves Game, he and Peko set up a cheat in order to make Team Baron the winner by making the opponent's Inves go berserk. After being discovered by Kaito however, Zack never dared to cheat again.

When Kaito quits Team Baron so that he can fight the Yggdrasill Corporation and not to put his teammates in danger, he gives Zack the Team's leadership and gives him a Sengoku Driver. When Oren is about to disrupt the Beat Riders' stage performance, Zack shows his heroic side for the first time in the series by risking himself to protect the Beat Rider dancers via becoming Kamen Rider Knuckle and assisting Gaim.

Arms
Like with the other Armored Riders, Kamen Rider Knuckle's forms are called Arms (アームズ). His Sengoku Driver plays American rock & roll music and emits a guitar riff sound before every transformation, while his personal Lockseeds are styled in pure English with a "Mr. [Adjective/Noun]" theme.

By pressing the Sengoku Driver's Cutting Blade down on the Lockseed after transforming from one to three times, Knuckle can activate one of three finishing moves based on his current Arms form: Squash (スカッシュ), Au Lait (オーレ), or Sparking (スパーキング).

Devices

 * Sengoku Driver - Transformation device
 * Lockseeds - Gives access to Knuckle's Arms changes, depending on which lock is used

Weapons

 * Arms Weapons - Weapons that come with the Arms:
 * Kurumi Bombers - Knuckle Kurumi Arms' personal weapons
 * Unnamed boxing gloves - Knuckle Suika Arms' personal weapons in Yoroi Mode
 * Kagematsu - Knuckle Matsubokkuri Arms' personal weapon
 * Donkachi - Knuckle Donguri Arms' personal weapon

Portrayal
Zack is portrayed by Gaku Matsuda (松田 岳).

As Kamen Rider Knuckle, his suit actor is Daisuke Satō (佐藤 太輔), who was also the suit actor for Kamen Rider Ryugen, Demushu and the Byakko Inves.

Legend Rider Lockseeds

 * The following Legend Rider Lockseeds are styled for Knuckle, as well as Bravo:
 * X - "Mr. Kaizorg!"
 * Amazon - "Mr. Wildman!"
 * Faiz - "Mr. Justifaiz!"
 * New Den-O - "Mr. Countdown!"
 * Beast - "Mr. Mayonnaise!"

Appearances

 * Kamen Rider Gaim
 * 1-6, 9, 12, 13, 15, 16, 18-21, 25, 26, 28, 30-40, 42, 44-47
 * Kamen Rider × Kamen Rider Gaim & Wizard: The Fateful Sengoku Movie Battle
 * Kamen Rider Gaim: Sengoku Battle Royale
 * Kamen Rider Gaim: Great Soccer Battle! Golden Fruits Cup!
 * Kamen Rider × Kamen Rider Drive & Gaim: Movie War Full Throttle
 * Kamen Rider Gaim Gaiden
 * Kamen Rider Gaim Gaiden: Kamen Rider Baron
 * Kamen Rider Gaim Gaiden: Kamen Rider Knuckle