To unlock a secret Darkling go to any pay phone inthe subway and dial 555-4263 (555-GAME). You willhear a message to confirm correct code entry.
Alien8d via Grady Simmons
When you are about to give up the Darkness guns andextract the Darkness, look at the Demon in the middle. Look behindhim in the bright light and you should see Jenny.
Alien8d via Grady Simmons
To defeat the Darkness in the castle (after you haveblown open the gate), go up to every chair you see in the mainroom. Each chair should prompt you to activate it. Note: Thisdoes give away your Darkness powers for the time being. Afterthe Darkness appears as a red/evil Jackie, put away any guns sothat just your hands are out. This should stop his attacks. Then,walk up to him without any guns out, and a prompt should appearto "Accept Him". The fight will then be over.