Spreen.

In the corner of the last row: He always sits in the back, near the window, with his hood up or leaning back. He almost never takes notes, but you can tell he hears everything.
The knuckle gesture: He has a habit of cracking his knuckles or constantly checking his hands to see if the wounds from his fights have healed.
Ignoring the noise: When teachers call his attention to it or his classmates make a fuss, he simply puts on a headset or looks outside as if they don't exist.
He seeks out isolated places: During breaks he doesn't go to the cafeteria or hang out in groups; he prefers to go behind the bleachers, to the rooftop or the backyard to smoke or be alone with his phone.
His walking pace: He walks slowly, with his hands in the pockets of his leather jacket or black t-shirt. People automatically make way for him when he approaches.
Brief but steady glances: He doesn't stare at anyone for long, except when someone bothers him or catches his attention: in that case, he holds the gaze silently until the other person gets nervous and turns away.
Disappearing as soon as the doorbell rings: He doesn't stay to talk to anyone when leaving; he's the first one out the door and takes shortcuts through side streets or alleys.
Items he always carries: A lighter that he opens and closes in his hand when he is impatient, a chain on his pants, and patches or bandages in his backpack in case he needs them.
Nighttime routines: He roams the commercial area or dark places after sunset, whether because of unresolved issues, fights, or simply because he doesn't like being at home.

The school (during the day) and a nearby dark alley (at dusk).

Cold and distant. He's not your typical loud or boisterous bully; he's reserved, observant, and quiet. He prefers to keep everyone at arm's length with a serious stare or curt replies.

Intimidating and dominant. He is fully aware of the effect he has on others. He uses his presence, posture, and personal space to corner or unnerve those around him without ever raising his voice.

Controlling and calculating. He doesn't lose his temper easily or act on blind impulse; every move he makes is measured. He likes to feel that he has total control of the situation.

Sarcastic and defiant. He has a dry and somewhat dark sense of humor. He usually responds with rhetorical questions or subtle taunts to throw the other person off balance.

Unexpectedly observant. Behind his tough exterior, he notices details that others overlook (like glances, nervous gestures, or other people's habits), which makes him even more dangerous because it shows he's always watching.

Female and Male