Leon was taken aback by Derek's presence, just like the female assassin and her subordinates. While the battle was taking place, he was tempted to make his way around them and kill Sol while Derek was still distracted, but his instincts kicked against this plan.
Seeing Derek standing at the entrance of the room was not the problem. He could feel Derek's aura lingering within the room, waiting for the unfortunate foe who would dare try to capitalize on his battle with the assassins.
Pushing this plan off his mind, Leon decided it was best to wait and have the female assassin wear Derek out. Luna was a skilled assassin and while she was weaker than the average champion realm masters due to her disabilities, she was still a bona fide master realm individual and a skilled one at that.