Atsushi Kawase works as an editor for novelist Subaru Mikazuki, persistently yet often futilely encouraging his reclusive charge to venture outside. He sports short light brown hair, brown eyes, and rectangular glasses, typically dressed in professional attire: a pink coat over a white undershirt with a purple collar, gray pants, and brown shoes.

Atsushi adores cats, particularly Haru, the cat Subaru adopted. He regularly visits Subaru's home specifically to see Haru, though he cannot have a cat himself due to his wife's allergies. Haru generally finds him noisy and disruptive, a dislike Atsushi seems oblivious to.

During critical moments like Subaru collapsing from exhaustion or facing severe weather, Atsushi rushes to his home to ensure his safety. He offers Subaru interpersonal advice, stressing the importance of expressing gratitude and maintaining connections despite past oversights, reassuring him that regret is common and future chances to show appreciation matter more.

Atsushi displays a childlike enthusiasm, readily joining in games with children visiting Subaru. This persistent kindness, coupled with his energetic nature, defines his supportive role in Subaru's slow steps toward social integration. Known details center solely on his editorial work, marriage, and consistent encouragement of Subaru's growth, without significant change in his own circumstances or personality.