So, I have an idea for this, but I want to make sure it's okay first:

I know that the monsters had only recently just appeared again, but maybe one of them(or their consciousness) escaped early, and tried to possess a teenager?

Unfortunately, the teenager was hella stubborn, and after a whole lot of effort, the monster, sick and tired of all of this but also stubborn, proposes an idea: "Why don't I live inside your mind for a while? Like, not take it over, like I've been failing to do, but just protect you when the other monsters break free? Because yes, more monsters are coming, and I [i]could[/i] save you from them as long as you accept the deal. In return, you have to [insert important task here]."

The teenager, reluctant but frightened at the mention of more monsters, decides to let the monster/consciousness stay, and an unspecified amount of years later because I don't know how old our characters are, the human gets their magic, but the monster stays in their mind because "the deal hasn't been paid off yet"(in reality, the monster has formed a bond with their human, and still wants to protect them, but is too tsundere to admit that xD).

Consider this like a first draft of an idea, as I don't really know many important details of the setting that would help me figure this out.