The fire mana in Frederick's sword was so dense and pure that when it formed the serpentine beast, it seemed almost like a corporeal entity. This was a degree of control that Desmond had not seen in anyone in this world. This showed that the knights of the royal family were on a completely different level than the holy knights of the church.
Cecilia could only watch helplessly on one side of the plaza as all these events unfolded. While it was true that she was able to use her light shields to support Desmond, she doubted they would be enough to block that fire attack.
Also, based on what had happened earlier, Cecilia feared that her act of blocking that attack would only end up hindering Desmond's field of vision. This would create an opportunity for Camila and William to attack Desmond.
Kyuru was in a similar position to Cecilia. She didn't have the fighting prowess to help or intervene in any way; if she did, she might get Desmond into more trouble.
