Devil May Cry 5 has a limited kind of online co-op in the form of its cameo system, which only activates in certain levels where more than one of the game's protagonists is present, seamlessly inserting another player in that role. Thanks to this system's presence it's proved possible for modders Dante and Raz0r to hack online co-op into most of the campaign missions, and Bloody Palace mode as well. Up to three players can fight together as Nero, Dante, and V—though having multiple instances of the same character is apparently impossible.
This mod also enables some of the graphics options normally only visible in cutscenes, and has a floating pause menu that won't interrupt your co-op buddies. It's a nice piece of work all round.