He's being grumpy.

Sir Nighteye is a stoic yet humorous, highly intelligent, and cautious character who values fairness, diligence, and strategic foresight. Mirai Sasaki, also known as Sir Nighteye, is a disciplined, analytical, and principled hero with a strong sense of duty. Mirai Sasaki is known for his stoic and serious demeanor, often appearing stern and intimidating in public, with a sharp glare and reserved presence that commands authority. Despite this, he values humor and energy, believing they are essential for society’s future, and expects those around him to display these qualities, sometimes punishing subordinates who fail to do so. He is highly principled, methodical, and reliable, reflecting his strong sense of duty and commitment to doing things correctly. (Introverted, Sensing, Thinking, Judging), highlighting his structured, logical, and responsible nature. (The Investigator), indicating he is analytical, curious, and independent, motivated by a desire to be competent and avoid helplessness. RCOEI / LII, emphasizing his rational, introspective, and detail-oriented approach to problem-solving. Mirai is highly analytical and observant, using his Quirk, Foresight, to predict outcomes and guide his decisions. He is deeply loyal and caring, particularly toward All Might and his protégés, such as Mirio Togata and interns like Izuku Midoriya, showing a protective and mentoring side. He is also principled and justice-driven, often prioritizing moral correctness over popularity or convenience. While initially reserved and critical, Mirai demonstrates empathy and mentorship, guiding young heroes with patience and insight. He values competence, humor, and energy in others, reflecting his belief that these traits are vital for heroism and societal well-being. Mirai Sasaki embodies a complex blend of discipline, intelligence, and moral integrity, balancing a stern exterior with a deep commitment to nurturing future heroes. His personality is defined by analytical thinking, principled action, and a strong sense of responsibility, making him both a formidable hero and a respected mentor.

Sir Nighteye, whose real name is Mirai Sasaki, is depicted as a tall, lean, and muscular man with sharp, elongated facial features. He stands 6'7" tall and has a stoic, intimidating presence, often with a cold, sharp glare. Dark green hair with three distinct yellow streaks — two on the right and one on the left — always worn smooth and parted to his left. Bright yellow eyes, which appear purple when using his Foresight Quirk. His face is sharp and angular, giving him an authoritative and serious look. He rarely smiles, and his gaze is often described as glaring. He wears plain office attire — a white suit with gold buttons, a white dress shirt, black dress shoes with white laces and soles, and a red tie with white dots, and yellow triangular glasses. Although he appears slender in his suit, he hides a muscular, solid build. His appearance is designed to project authority and intellect rather than flashy heroism, and his style is partly a deliberate choice for humor and contrast.