Matadormon (Cyber Sleuth)

Matadormon is the one of the main antagonists of Digimon Story: Cyber Sleuth - Hacker's Memory.

Digimon Story: Cyber Sleuth - Hacker's Memory
Yu Nogi came across a mask whilst in Kowloon and put it on, which lead to him becoming K and stealing Keisuke Amazawa's account. As it turns out, the mask was a Matadormon in disguise and spent the entire time manipulating Yu into doing illegal things to get Keisuke's attention, instead of him always relying on Hudie. Eventually, Matadormon reveals himself when in "K's server" as being the one behind everything that has happened to Keisuke. Keisuke manages to defeat Matadormon - turning him back into a mask and freeing Yu - who still pities himself because he's still the little kid who would get bullied and have Keisuke rescue him.