From Wikimon

An Illegal is a special type of Digimon that appears in the manga Digimon Next. These special types of Digimon are marked by a hexagonal symbol somewhere on their body. When a Tamer logs into the net game, they have to go through certification so the tamer's information can be inserted into the Digimon. When this happens, there is a chance that an illegal could be made.

They are said to have special powers. These include:

-Protect: creates a hexagonal shield that blocks attacks. These are not indestructable, however.

Currently known Illegal Digimon[edit]