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 received 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 transformed 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 Kota went to Team Baron wanting them to participate, but Kaito refuse. After they left, Peco 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 Peco looked at the injured citizen as they saw 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.

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 Peco 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 Peco'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. Unlike other Armored Riders, Knuckle's Sengoku Driver does not have a Rider Indicator on it due to it being a mass production type.
 * Arms Weapons:
 * Kurumi Bombers - Knuckle Kurumi Arms' personal weapons
 * Lockseeds - Gives access to Knuckle's form changes, depending on which arm the lock is.

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

As Kamen Rider Knuckle, his suit actor is currently unknown.

Legend Rider Lockseeds

 * Despite not having the opportunity to utilize the Legend Rider Lockseeds on-screen, the following toyline-exclusive Lockseeds are styled for both Knuckle and Bravo:
 * X
 * Amazon
 * Faiz
 * Faiz Blaster Form
 * Beast

Appearance

 * Kamen Rider Gaim
 * 1-6, 9, 12-13, 15-16, 18-21, 25, 26
 * Kamen Rider × Kamen Rider Gaim & Wizard: The Fateful Sengoku Movie Battle
 * Kamen Rider Gaim: Sengoku Battle Royale