The Darkness:
Place an Evil Character from hand face down here. If an opponent begins a rescue attempt and chooses a blocker (or is unblocked), you may reveal this evil character. Blocking player may add it to the battle. Otherwise return it face down.
Based on the bolded phrase, The Darkness may need to be the first one activated, but I'm not sure. Does The Darkness work if a CTB card is used mid-battle?
The site guard may face a similar situation, depending on what was used to place it.