Zaku-01

Zaku-01 is a random duelist who is well acquainted with the main crew and is great friends with Cato. Zaku's appearance is based off of the mobile suit "MS-06 Zaku II" from the Gundam franchise. While they are based off of a villain, Zaku in DMDR is not really a bad guy.

Introduction
Zaku joined the crew very early, right after everyone was transported to the Discord dimension. Zaku stayed with the crew for a while, then they split up to find the rest of the crew faster, although Zaku ended up completely alone, and could not find anyone.

The Thunder Dragon King Arc
The crew has found server members either unconscious on the floor, or acting aggressive, mindlessly dueling other people.

While these people were being mind controlled by someone, if their opponents would lose to them, they would become mind controlled as well, acting like some sort of "virus".

Since Zaku never found his way back to the crew, he just wandered around aimlessly without knowing there was an actual threat afoot. In a flashback scene that was shown from a footage that was recorded by Zaku, he actually encounters the Thunder Dragon King right away, who was disguised as a regular server member. As Zaku is a hectic person, he immediately pulled his gun on the King without even knowing who he is. Zaku noticed that the "server member" was not too bothered by the gun being pointed at them, so Zaku scans the duelist for further information. After the scan, his system warns him that there is no information about them, and they had a lot of bloodlust. Because the member showed no information, Zaku quickly exposes the member does not originate from the server dimension. After the realization, the King quickly transforms and threatens Zaku. The quick witted Zaku tries to interrupt the King's transformation by shooting at them, but the King deflects the shots, and fires a shockwave at Zaku, which ended up breaking Zaku into pieces. This entire interaction was recorded by Zaku before he was destroyed, and the footage was viewed by the original crew once they found Zaku's broken body.