The purple-masked person is one of the colonels in Ghost Nebula and has the code name Omega. He is a Realizer in the Planetary phase, and his power is one level higher than Theta.
Isao and the others were alert when he saw Omega appear, and they were shocked after hearing his words. That person intends to kill them and capture Chizen for an unknown purpose.
If they were in their peak state, they could confidently fight against Omega. But because of Hydra and Theta's attack, Subaru and the other two leaders were seriously injured, and they would lose if they had to fight him.