Arcadiamon (Cyber Sleuth)

 is the one of the main antagonists of Digimon Story: Cyber Sleuth - Hacker's Memory. It is a Digimon who was imprisoned on Underzero, then escaped and caused havoc in the Real World and EDEN.

Description
Arcadiamon was trapped in the Underzero, and after being accidentally freed by Ryuji Mishima, its sole desire is to kill and absorb Digimon so that it can get more and more powerful, so it can digivolve into its more powerful forms and cause destruction wherever it goes. After Ryuji decides to get revenge against all Hackers in revenge for his best friend Chitose Imai succumbing to EDEN Syndrome, Arcadiamon and Ryuji team up to kill countless Digimon and Hackers to achieve both of their goals. Despite this, Arcadiamon and Ryuji become close friends and after Arcadiamon's defeat to Keisuke Amasawa, Ryuji apologises for causing his partner to turn out the way it did, and Arcadiamon decides to try and help Ryuji the only way it knows how, by killing Ryuji so he doesn't have to face the wrath of his friends. suddenly appears and blocks the attack however, so Arcadiamon fails in its final act and succumbs to its wounds.

Fiction
After summons a large group of Eaters to attack all the Hackers in Underzero, one of them is about to absorb Ryuji Mishima, however Chitose Imai pushes Ryuji out of the way and takes the blow instead, succumbing to EDEN Syndrome. This sends Ryuji into a deep depression, and as he wanders Underzero he accidentally frees Arcadiamon (In-Training), who attacks Ryuji. Ryuji's partner protects Ryuji and takes the blow however, and is absorbed by the In-Training Digimon. Arcadiamon then becomes Ryuji's new partner Digimon as the duo team up to kill Digimon and Hackers to achieve their own goals.

Arcadiamon (Rookie)
Arcadiamon (Rookie) is Arcadiamon's Rookie form.

Soon after absorbing, Arcadiamon digivolves into its Rookie form. On Kowloon Lv.1, Ryuji orders him to attack Takkun and Miporin.

Arcadiamon (Champion)
Arcadiamon (Champion) is Arcadiamon's Champion form.

When Keisuke Amasawa is about to lose to K and his, Ryuji and Arcadiamon (Rookie) easily defeats and absorbs it. They then go on a rampage, absorbing all sorts of Digimon all over Eden and the real world, in an attempt to eliminate all hackers in the world as revenge for an Eater eating Chitose Imai. After absorbing an IceDevimon it digivolves into its Champion form. Keisuke arrives to confront Ryuji but he just walks off.

Arcadiamon (Ultimate)
Arcadiamon (Ultimate) is Arcadiamon's Ultimate form.

Fei Wong Tomoe Ignacio, Date, Keisuke and Erika Mishima come up with a plan to ambush Ryuji, in which they stall him in Eden and then catch his body in real life whilst he's logged in after Erika hacks to find his position. Fei stalls Arcadiamon whilst Keisuke and Date track down Ryuji in the real world, however it appears Ryuji was one step ahead of them and the signal sends them to a random hacker instead. When Fei is informed, the Ryuji/Arcadiamon in front of them vanishes, with it merely being a fake. Upon returning to the Hudie base, Ryuji walks in and states that Fei is first on his list to kill, as she was responsible for Chitose being attacked by an Eater, as it was Zaxon's plan to go to Under Zero and that after he's killed Fei, Yuugo Kamishiro was next. Fei is pissed off at the threat to Yuugo's life and her comes out to help Fei. Erika attempts to calm down her brother but Ryuji just screams at her, saying that him losing all his friends, his family and even Erika herself is her fault. Arcadiamon then digivolves into its Ultimate form, destroys the Hudie base and flies out into the sky, with Keisuke - who was on his way back - seeing the duo flying off. TigerVespamon manages to save the lives of Fei and Erika whilst the building collapses.

Arcadiamon (Mega)
Arcadiamon (Mega) is Arcadiamon's Mega form.

Keisuke eventually finds Ryuji and Arcadiamon (Ultimate) in Akihabara. Keisuke tells Ryuji that Chitose is now awake, as he and Erika had hacked an Eater, gone to the Eater World and acquired Chitose's data, allowing them to restore him out of his Eden Syndrome. Ryuji doesn't believe Keisuke and Wormmon and then states the two had betrayed him and attack the duo. After Keisuke manages to defeat Arcadiamon, Ryuji flees to find more Digimon to have his Arcadiamon eat to power back up. At this point, Arata Sanada in his forms arrives and the three team up, searching the Akihabara Digital Shift. They find Ryuji has caused it to be an endless loop, so hack the environment and find that three random hackers have been mind controlled to cause it. After defeating them in a dominion battle, the area is restored to normal and the three find Ryuji with Arcadiamon eating multiple Digimon to power back up. Ryuji then sees Arata and states that Keisuke has betrayed him after all, since he is now teaming with the guy who betrayed Ryuji by leaving Jude. Ryuji and Arata argue over it and when Arata states that it's Ryuji's fault Jude ended because he could've just continued without Arata there, Ryuji's anger causes Arcadiamon to digivolve into its Mega level. Keisuke and Arata manage to defeat Arcadiamon, leading Arata to reveal he left Jude because in the past he got friends killed and losing in Under Zero brought those memories back. He didn't want to lead any other friends into getting killed again and that he knew Ryuji would force him to be the leader again and that Ryuji would've been a good leader for Jude, due to how well he had been leading Hudie until he and Arcadiamon started murdering hackers. Arata then leaves, with Ryuji in shock. Suddenly, Chitose and Matayoshi arrive with Ryuji being amazed that Chitose is awake like Keisuke had claimed. The dark aura suddenly vanishes from around Ryuji as he's happy to see his best friend alive. Just as Ryuji states he's done too much to ever return to Hudie though, Chitose is pissed and throws Wormmon at Ryuji, screaming at him about how Ryuji has been an idiot in his actions and that attacking Zaxon pretty much screws over Hudie. After getting everything out, Chitose asks for Ryuji to return though before he can, Arcadiamon wakes up and attempts to murder Chitose. Ryuji dives in the way, protecting him in the same way Chitose had done before, leaving Ryuji to take the blow and get absorbed by Arcadiamon, who then flies off with Keisuke, Chitose, Wormmon and Matayoshi shocked. Ryuji is able to talk to Arcadiamon whilst inside him with Arcadiamon stating that he will destroy Ryuji before anyone else can and before his friends abandon him for what he has done. Ryuji apologises for making his partner the way he is, though Arcadiamon still tries to kill Ryuji. suddenly appears and blocks the attack, with Ryuji happy to see his old partner survive. They then decide they need to return to Hudie.

Arcadiamon (Ultra)
Arcadiamon (Ultra) is Arcadiamon's Ultra form.

After having absorbed Ryuji, Arcadiamon is shown appearing from inside a lake in Odaiba in its Ultra form. Keisuke and Erika arrive to try and fight it but find their Digimon unable to hurt the building sized monster. With no way to win, Wormmon wants to flee, only for Erika to send her Eater Bits to attack it instead. This slows down Arcadiamon, with Keisuke's Digimon now able to hurt the giant Digimon. They manage to defeat it after a long fought battle. After being defeated, Arcadiamon explodes with Ryuji and being released in the air. Cyberdramon flies Ryuji down to the bridge, with Makiko Date charging in to arrest him, though changing her mind when seeing him injured, instead getting him am ambulance. Erika then collapses due to overexerting herself, which leads to her Eater Bits attacking her, merging with her body to create Eater Legion and then flying off, with everyone shocked at what just happened.