Sir Nighteye is a lean but muscular man with a clean, well-kept appearance, wearing regular office clothes (a light gray suit with a red polka-dotted tie) while on duty. He wears a pair of glasses and appears to be always glaring. His hair is of a dark greenish color, save for a few blond strands located on his bangs as well as blond eyebrows.
Unusually for a Pro Hero, Nighteye appears to prefer plain office clothes instead of an elaborate Hero costume. His clothes, however, seem to be quite durable as he uses them during battles and they don't show any sign of damage or tearing.
According to his character bio in Volume 14, Nighteye was designed with the intention of invoking the stereotypical image of a Japanese man.
After fighting with some villains, Nighteye ends up grievously injured and goes to the hospital. At the hospital, doctors tell his visitors that he won't last. All Might who was called by Bubble Girl walks into the room, as Nighteye remarks on how it fitting it is for his mentor to visit him one last time on his deathbed. All Might laments on not visiting him and begs his old friend to pull through so he could make amends.
Nighteye tells Izuku that he was wrong about him as he was able to change the future after Izuku begs him not to die. Nighteye doesn't dwell on his fate as he states Izuku has shown him a way to change the future when he previously failed to find a way to stop All Might's impending death. Mirio ran into the room and frantically begged his mentor not to leave, as he still had much to learn from him. Nighteye then comforted Mirio by using his Quirk one final time to tell him he'd be a great hero in the future. Nighteye gives his pupil his final words before succumbing to his wounds and passing on.