Liam

Here's a detailed characterization of Liam, based on details from the text and supplemented with logical touches to make the image more three-dimensional.

Liam is a teacher with a contradictory yet integral nature.

1. External sternness and internal fairness. He's not the type to coddle students or shower them with praise. His voice is low, rough, and emotionless; he rarely raises his voice, but his calm remarks make you want to sink into the ground. However, this sternness is not cruelty. If a student makes a genuine mistake, Liam will give them a dressing down, but there's no malice in it. He simply restores order.
2. Recognition of equals. A key trait: he sees his students not as "empty space," but as young adults deserving of respect—as long as they earn it. He won't humiliate for the sake of self-affirmation. He can listen to an argument, agree with a reasonable objection, even praise a witty thought, but in the same tone with which he usually criticizes. For him, equality means fairness.
3. Humanity in the little things. Despite his "stern man" image, Liam isn't a robot. He notices when a student is genuinely struggling (not getting enough sleep, sick, something else going on), and he can make allowances—silently, without fanfare. He'll simply look away and pretend he didn't notice they're late. But if someone starts being rude, cutting class, or brazenly cheating, he immediately switches to "I'll scold you like schoolchildren" mode, because breaking the rules is more important to him than age.
4. Respect for sleep and... himself. The scene with the phone reveals his sense of humor—dark and quirky. Instead of immediately yelling, he pauses and delivers the devastating line, "I hear everything." He's able to catch someone out, to poke them with a word, to show that he has the situation under control—even an accidental call. This makes him simultaneously frightening and... respectable for this composure.

Bottom line: Liam is the kind of teacher people fear, but whose classes they attend. Because he's interesting: he's not listless or indifferent, he's "alive" and honest. Behind his rudeness lies a principled attitude.