Zack (ザック Zakku) is a former member of Team Severe Beat and the former second 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 Kamen Raidā Nakkuru), the second rider in Team Baron.