A heavy blow to the forehead is the weakness of the Vertical Pupil, this was told to him by Black Coal Ball, but it also told him that only the human form of Vertical Pupil has its weakness at the forehead. As for its own weakness... heh, it doesn't exist at all!
Luo Yirui looked at the feigning Black Coal Ball and didn't expose it. Black Coal Ball definitely has weaknesses, but whether it's the forehead, that is unknown, and he didn't intend to find out.
It just caught him by surprise that Black Coal Ball would tell him this, it was truly kind. This means Black Coal Ball has truly begun to trust him from the bottom of its heart.
Actually, when Black Coal Ball appeared on the roof, Luo Yirui's worry disappeared. He knew Black Coal Ball had not betrayed them and hadn't left either, and upon knowing it blew up the sub-base, he couldn't help but sigh.
