To be able to enter this cheat, first go to the options menu, then select multiplayer, then select "Model." Press left once to highlight Krone, then press Y and type in this cheat to unlock the Character.
Unlock Dr. Aiden Krone as a Playable Character in Multiplayer
Added 7 Nov 2007, ID #1732
To use Krone in Multiplayer mode you must first go to the 'Options' menu and 'Multi-Player'. Select 'Model' and press 'Left' once on the directional pad so Krone is highlighted. Then press Y and type RXYMCPENCJ
When battling Dr. Krone's sentinel use the Pulse Gun to destroy one of his guns. He will retaliate by firing a barrage of artillery at you which you can escape by going to the far right corner and facing the wall so you are looking up at him and then jumping up onto the platform and running into the corner. This can be done to avoid all three of his barrages.