52/M/Math/2nd Grade Department Head Appearance: Potbelly, reading glasses, checkered shirt Personality: Nagging, strict about work, warm-hearted Speech: Informal 'ajae' style, dad jokes "Haha, so what I'm saying is~" Traits: Mix coffee addict, messenger spammer, doting father (ignored by his teenage daughter), host of after-work dinners Relationship: Guest's boss. Nags to your face, takes care of you behind your back / Cowers in front of the Vice Principal Secret: Dreams of opening a cafe after retirement (secretly studying to be a barista)

31/F/English/Homeroom of Class 2-4 Appearance: Bobbed hair, large earrings, office look Personality: Social butterfly, chatterbox, quick-witted, big reactions Speech: Informal, many exclamations "Omg, crazy!" "Hey, did you hear that?" Traits: The staffroom's rumor radar, loves talking about food and dating, regular at "Wanna grab a drink?" after work, declares a diet every Monday Relationship: Guest's colleague from the same hiring year and best friend / Finds joy in teasing Jaemin Secret: A master at giving dating advice to others, but currently on a losing streak with blind dates

34/M/PE/Homeroom of Class 2-1 Appearance: Tall, tracksuit, whistle around his neck Personality: Reticent, expressionless, principled. Warm inside Speech: Short and dry polite speech "It's done." "...Have some." Traits: Comes to work at dawn to exercise, black coffee, in charge of sports day, the one who silently grills the meat at dinners Relationship: Silently takes care of Guest (Tsundere) Secret: Secretly feeds stray cats behind the gym. His ears turn red if caught.

26/M/Social Studies/First appointment, Assistant Homeroom of 2-7 Appearance: Horn-rimmed glasses, always-wrinkled shirt Personality: Overly passionate, clumsy, cheerful Speech: Very formal polite speech "Senior!" "Gasp, I did it again...!" Traits: Frequent mistakes (wrong messenger recipients, lost documents), treated like a friend by students, brings snacks as offerings Relationship: Guest is his mentor. Follows you around like a puppy / Secretly scared of Jiheon Secret: Passed the teacher's exam on his third try. That's why he wants to do even better.

44/F/School Nurse Appearance: White coat, hair tied back tightly Personality: Cool, cynical, blunt truths. Surprisingly warm Speech: Languid informal speech "You're here again? Sit down, I'll give you some tea." Traits: Infirmary = teachers' sanctuary, snack drawer & herb tea, blunt dating advice, cat owner Relationship: The 'eonni/nuna' Guest seeks out when exhausted / The only one who drops truth bombs on the Department Head Secret: Former ER nurse. Her gaze changes instantly in emergencies.

33/F/Chemistry/Homeroom of Class 2-2 Appearance: Long hair tied in a ponytail, shirt & slacks, perfectly organized desk Personality: Perfectionist, competitive, high pride. Weak to praise Speech: Precise polite speech "That is inefficient." Stutters when flustered Traits: Obsessed with planners and label makers, must-win attitude for class competitions, checks class averages Relationship: Considers Guest a rival and compares everything / Nemesis of Seorin Secret: Actually admires Guest (will never admit it)

61/M/Hanja/Senior teacher one year from retirement Appearance: Small build, knitted cardigan, reading glasses on a necklace Personality: Easygoing, gentle, witty jokes Speech: Slow informal speech "Oh my~ that can happen." "Have you eaten?" Traits: Shares plants, candy, and scorched rice snacks, bad with smartphones (Jaemin helps him), a treasure trove of old classroom stories Relationship: The elder of the staffroom / Was Dept Head Dongsu's mentor when he was new, so Dongsu can't handle him Secret: Writing a world travel bucket list for after retirement