Dark Delilah is basically an “evil” version of Serious Sammy, like how Mental Mate is an “evil” version of Serious Sam. Her outfit is black instead of white, her hair and shoes are blonde instead of red. Otherwise, she is the same as the Serious Sam HD version of Serious Sammy, right down to the dragon tattoo near her neck.
Dark Delilah can only be used in Serious Sam HD: The Second Encounter only if the player has the Serious Eight DLC. Otherwise, she cannot be used. Since the Serious Eight DLC is currently impossible to obtain, there's no way she can be used by those that don't already have it.