Arthur resembles a black leech-like creature with a glowing red eye and a dark cloud around its body.
Powers and abilities
Arthur is able to scan any unstable code within any digital being, then copy and transmit it throughout the area. This ability is not limited to the physical composition of the target, as Arthur can even scan the target's emotional insecurities, as it scanned Ralph's grief over losing Vanellope; however, the products of its viral proliferation depends on the error in question, given that scanning Vanellope's deteriorating glitch destabilized the structure of Slaughter Race while scanning an emotional Ralph generated hundreds of Ralph clones that embodied his negative traits. There's a known fact that when Arthur scanned Vanellope, her insecurities are at 30% meaning that Ralph's insecurities was 100% after losing Vanellope.
Role in the film
After Ralph earned enough money to buy the wheel for Sugar Rush, he waited for Vanellope, but she didn't show up. He tried to call her, but it accidentally turned on itself on mute. He overheard Vanellope tell Shank she wanted to stay in Slaughter Race. Ralph did want to lose her friendship so he asked Spamley was there a way to slow down the game. Spamley tells there's only one way so he takes Ralph over to the Dark Web to meet Double Dan.
When Ralph tells Double Dan he wants to slow down Slaughter Race, he presents his latest virus named Arthur. He warned Ralph it can scan any defect in any game, then copy it and spread it around parts of its surrounding area. Double Dan also warns Ralph not to let Arthur escape from the game or it would cause chaos across the Internet.
Ralph then releases Arthur inside Slaughter Race during Vanellope's race with Shank. Arthur is drawn to Vanellope's glitch and proceeds to transmit her insecurity throughout the game. As a result of the unstable glitch, the game deteriorates and crumbles around the racers, though Shank and Ralph carrying the unconscious Vanellope flee as the game reboots. When Vanellope comes to, she discovers Ralph was the one who released the virus. Enraged, Vanellope takes Ralph's medal and throws it down to the Dark Net, and storms off declaring they are no longer friends. While a heartbroken Ralph heads down to retrieve his medal, Arthur, emerging from Slaughter Race's wreckage unscathed, detects Ralph's emotional meltdown prompting it to copy the bad guy's insecurities and launches the faulty code across the Internet in the form of mindless Ralph clones.