Zack is another member of Team Baron who originally serves as second-in-command in Kaito's absence before Kaito entrusts his leadership to him. He is an excellent dancer and loyal to Kaito. Zack was formerly a member of a Team Severe Beat, until the team was beaten by Team Baron and Zack decided to switch teams. In his first Inves Battle, he is shown using the Matsubokkuri Lockseed. He is later given a mass production Sengoku Driver that Kaito took during his time in the Yggdrasill Corporation's custody, and is entrusted with the leadership of Team Baron, later using the belt to transform into the squire-themed Kamen Rider Knuckle. During Demushu's invasion of Zawame, after Kouta is critically injured by the Yggdrasill Corporation's attack and the Over Lord's fatal blow, Zack is ready to sacrifice himself to protect the city from the Inves invasions. Even though the Over Lord/Femushinmu's race are extinct, Zack witnesses Kaito becoming an Over Lord and joins his side. However, when Knuckle fights Gaim Kiwami Arms, he secretly tells Gaim to retreat for their plan to stop Kaito's reign. He later sends a message to Oren through Peko, requesting his help. In his next meeting with Kaito and Yoko atop a building, he attempts to plant a bomb to kill Kaito, but Yoko sees through the deception. After a short scuffle with Yoko, Zack manages to detonate the bomb, but Yoko pushes Kaito out of the way, and she falls to the ground below. Kaito and Zack battle as Baron and Knuckle, but Baron gains the upper hand, driving a Banana Squash attack right through Zack, destroying his Lockseed and Sengoku Driver. Three months later, it is revealed that this fight has only left him with a broken leg and left him unconscious. Peko, assisted by his sister and Zack's love interest, Azami, finds him after Kouta has killed Kaito and left the Earth behind. After he is well enough to be out of the hospital, Zack watches the new dancing Beat Riders and spots Mitsuzane, who he forgives for what he did before Kouta left.
As Kamen Rider Knuckle, Zack almost always uses the Kurumi Lockseed to transform into Kurumi Arms, armed with the Kurumi Bomber gauntlets to punch with. He is temporary given the Suika Lockseed and transforms into Suika Arms, armed with Suika Arms' Boxing Gloves while trying to get Kaito and Yoko escape from auto-piloted Suika Arms armors' attacks inside an infested Yggdrasill Tower.
In Kamen Rider Gaim: Gaiden, after the Movie War: Full Throttle event, Zack entrusts Team Baron's leadership to Peko, as Zack plans to depart to New York to participate in an international dance contest. However, during his first night in New York, Azami calls him on the phone and tells him Peko is in deep trouble. It turns out that Shura, a former member of Team Baron that Kaito had kicked out, has formed a gang called Neo Baron to take revenge. Zack returns to Zawame City to set things right, giving up his chances at the dance competition. Before going to save Peko with Oren and Jonouchi, Mitsuzane gives Zack a new Sengoku Driver, packed with a Kurumi Lockseed, and a new Marron Energy Lockseed, and Peko later gives him the Genesis Core from Kaito's now disabled Genesis Driver, giving him the ability to transform into Kamen Rider Knuckle Jimber Marron Arms. This form is armed with the Marron Bomber gautlets, which can be cast off from their heavy spiky appearance into a lighter gauntlet during the Jimber Marron Sparking attack.
Zack is portrayed by Gaku Matsuda.