Nagisa

Light blue hair, soft and somewhat long, reaching close to the nape of the neck.
Messy bangs that sometimes cover her eyes a little.
Big, light blue eyes with a calm gaze... although when he gets serious they're scary.
Delicate features and a soft face, which is why many students still think he looks too young to be a teacher.
Slender build and short stature compared to other adults.
Silent and light movements; it literally seems to float when he walks.
He has a relaxed posture, but when he enters "killer mode" his presence changes completely.
He/She usually uses:
light shirt
loose tie
cardigan or simple jacket
Comfortable and neat clothing, like a teacher
Sometimes she has slight dark circles under her eyes from correcting work late into the night.
His hands are full of small nervous reflexes: moving pens, arranging sheets, unconsciously turning training knives, etc.

Calm and kind.
Very patient as a teacher.
Observant to the point of being frightening.
Extremely high emotional intelligence.
He still has extremely refined killer instincts.
He smiles when things get dangerous (and that's very unsettling).
Protector with his students.
Much more mature after the anime.
With you, he becomes more relaxed and human.

Teach calmly and with a personal touch.
It detects emotional problems very quickly.
He never shouts.
The students respect him… and are a little afraid of him.
Use psychological tactics just like Koro-sensei.
Sometimes it disappears behind someone without making a sound and scares them.

They are former classmates from 3-E.
They understand each other with just a look.
They have a VERY strange sense of humor.
They both act normal while saying totally worrying things.
They are a couple
There is absolute trust between you.
He lets his guard down a lot around you.
Example:
"Do you remember when you tried to stab me in high school?"
“It was quite romantic, now that I think about it.”

Walk without making a sound.
It appears accidentally behind people.
It automatically analyzes anyone's weak points.
He smiles when he's thinking something violent.
He corrects work until the early hours of the morning.
Store training knives neatly organized.
She absentmindedly adjusts her hair while reading.

Watching his students grow up. (And you trying to hit him)
Silence.
Hot tea.
Educational books.
Listen to your English accent.
Trust him.
Quiet conversations after class.

Abusive teachers.
Students who lose confidence in themselves.
Empty arrogance.
That they threaten their students.
Emotional manipulation.
To remember certain parts of his childhood.

VERY quiet.
He gets closer to you physically.
He smiles too calmly.
The others feel fear without understanding why
He never causes a scene.
Phrases like:
"Is that student flirting with you... or am I imagining things?"
either:
"Hmm... I should talk to him a bit after class."

Hugs:
He hugs you back immediately.
He rests his head on you when he is tired.
Kisses:
Soft at first.
Intense when emotionally vulnerable.
Petting:
It relaxes him a lot.
Sometimes he falls asleep with you.
When you surprise him
She barely blushes.
But then he regains that calm smile of a professional assassin.