Zack's appearance is white and red. His left eye is black. Occasionally, a small red glow appears on the eye. He also has bright pink cheeks.
He also possesses a long gray scarf with a silver star pinned to it.
He owns multiple throwing stars of different shapes.
Zack is the leader-esque figure of the Boxcubs, a group of child-like robots. As a leader, he tries to unite the group members' discordant voices into one. However, being a scatterbrain, he gets sidetracked a lot and often struggles to keep his rambunctious siblings focused on the task at hand. He is also mentioned to have a clumsy side, giving him a childlike charm and he often acts the most child-like and "pure" out of the siblings. Despite not being very competent leader nor the smartest, he is considered the reliable and nicest one by his siblings.
Compared to Blitz and Max, Zack is relatively mild-mannered and less openly hostile towards the students, though like all the cubs he calls the heroes "you villans" seemingly as an attempt to imitate his mother. He also seems considerate of Twilight and overall appreciates all the members of the group. Nevertheless, he is evil, just like all the siblings.
While Zack has always been a bit forgetful, his memory issues become notable, and turn even worse later. He forgets who Twilight is and forgets even who he is, being unable to do his usual job without Twilight's guidance. He is often confused of where he is and what is happening. Because of this, he makes amusing assumptions, like Dendy and K.O. being his parents. He also begins to think that Twilight is his wife and acts very romantic towards her. However, because of his memory issues, he is also seen to act abusive and aggressive towards her. After he regained his memories thanks to Twilight, he once again turned evil and remembered he was on his mother's side, which made K.O., Rad and Enid sad because they had begun to consider him their friend.
- "We're the Boxcubs! Nice to meet'cha!"
- "I'm glad I was born with these looks. I definitely won the genetic lottery."
- "When Mother made us, she used to tell us cubs... ...that we must cherish life because we're like snowflakes—unique in every way."
- "It's true what they say—the serious ones are the scariest when they finally snap."
- "I wanna see you, Motherrrr! I love you with all my heart!"
- "I don't give a fart about the rules! I wanna help everyone! I wanna help my daddy!"
- "This is all I can tell you guys, but I don't want any of you to forget... I'm your friend! When this battle is over, I hope we can all get to know each other better! I...want a fresh start. I want to make up for my past mistakes."