I think the monster is either Yue or nancy the realtor. And Bradley for the Familiar Voice for dramatic purposes. Does it make sense? No. Do i have any evidence? Of course not. But we have nominated every other character, all characters have been presented for one of the two options, sometimes for both. Someone has to get it right even if it is by a bruteforce method.
EDIT: Wait wait, we had forgot about one option for the Monster. Hear me out. MC father, military stationed in germany. Military uses some heavy computer tech, it could explain the flawless hacking, it could explain MC pasion for IT as he grow watching his daddy doing computer things, and explains The Monster motivation for caring for the MC, he is protecting his son
By the way, im still joking.