Zack

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

- Zack's roll call

Zack (ザック) is 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 shown using the Matsubokkuri Lockseed. He later receives a mass production Sengoku Driver and a Kurumi Lockseed from Kaito during his capture and escape from Yggdrasill. With the Sengoku Driver and Lockseed he received, 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
As Team Gaim tries to convince the Beat Riders to participate the dance event, most of them deny the offer thanks to Kaito. Mai, along with Kouta went to Team Baron wanting them to participate, but Kaito refuse. After they left, Peko reveals his purpose of joining Team Baron as Kaito decides to quit and gives Zack the Sengoku Driver before he leaves without telling his reason to the team. Later, Team Baron went to Team Gaim's garage as they want 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. Zack transforms into Armored Rider Knuckle and assists Gaim. While fighting, the rest of the Beat Riders (less Team Red Hot) joins in the event as they are about to dance. However, the music stops playing, but it didn't stop them from dancing. While fighting the Inves, Baron appears to assist Knuckle before they finish off the Inves together for one last time.

Later, Knuckle fought the two Inves in Zawame. After defeating them, Zack and Peko looked at the injured citizen as they saw Helheim's plants growing from their wound, making the citizens panic and forces them to back off. Oren arrives along with Hideyasu and still mocks the Beat Riders until three Inves appears.

Knuckle helped Gaim and Ryugen fight the Elementary Inves before finishing them off.

Mai inform to the Beat Riders about Yggdrasill's plan as she decides to reveal in public. However, a horde of Inves appears as Zack and Mitsuzane fights them. Later, he found Kaito and learns that he was working for Yggdrasill, which made Zack worried about humanity's future.

Zack would join 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 would 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 by Kaito a Suika Lockseed, 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 a civilians who were attacked and caught by an Inves army. He almost close to death by uncontrolled Gridon Suika Arms after giving Gridon a Suika Lockseed.

When Kouta finds out about Mitsuzane's true color of using Beat Riders as a scapegoat, being a brother of Yggdrasill's overseer but betrays him, as well as allied with a villains like Ryoma, Sid and then currently Over Lords to the Beat Riders as what Kaito and Minato trying to Kouta about, Zack becomes enraged to here Mitsuzane's allegiance.

Zack later aids Kouta in fighting Duke, but was easily defeated by the New Generation Rider. With the interference of Baron, Ryoma informed about his plan to others. Zack and some others does not appear to trust him, just as what Minato said about her experience with Ryoma.

Despite not trusting Ryoma, 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 will search for Kouta.

Personality
Zack serves as second-in-command in Kaito's absence. He is an excellent dancer and loyal to Kaito. 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. Their actions were detected by Kaito who furiously broke Peko's slingshot, restricting them from cheating 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).

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 currently unknown.

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!"

Appearance

 * Kamen Rider Gaim
 * 1-6, 9, 12, 13, 15, 16, 18-21, 25, 26, 28, 30-40, 42
 * Kamen Rider × Kamen Rider Gaim & Wizard: The Fateful Sengoku Movie Battle
 * Kamen Rider Gaim: Sengoku Battle Royale
 * Kamen Rider Gaim the Movie: The Great Soccer Match! The Golden Fruit Cup!