While Devil May Cry 5's cameo system may be a considered something of a co-op mode, it is limited in that it's only present in certain areas of the game, and there's limited options in terms of who you end up playing alongside. 

Fear no more though, as true co-op gameplay is here thanks to a brand new mod created by Dante and Raz0r, which enables full co-op in the majority of the game's campaign.

The mod lets two or three players fight together, as Dante, Nero or, of course, V, but you'll need to fight among your friends as to who will be who, as you can't have more than one of the same character. 

A nice bonus though is that the mod also works in Bloody Palace mode, and even goes so far as to modify the pause menu to prevent interrupting the flow of gameplay for the other players. You can check the mod out over at Nexus here.