At first, I thought that the Demon who tried to pass itself as a Human was either the Demon that Tina saw in prison or the one who infiltrated the Academy, but… I was wrong.
After the Demon got hit by my Light Attribute Mana Arrow, it turned back to its real appearance. I observed the Demon's transformation and realized that the Demon was, in fact, the original Demon that appeared in the novel.
That Demon was strong. Zaiden wouldn't be able to take care of it himself. If he tried to fight it with his current strength, he would be trashed or insta-killed.
I looked around again and noticed two Demons exiting the dungeon. Once again, the Demon described by Tina wasn't among them.
'Is it hiding inside the dungeon?' If that was so, then he was the one who controlled the monster using a monster breeder. Possibly, he also controlled the boss monster, if there was any, that was.