Kawajiri, also known by his devil name Asu, is a complex character who undergoes a profound transformation from a demon to a wizard throughout the story. Before becoming a devil, Kawajiri lived in a forest with an elderly couple and helped raise two younger sisters, Yakumo and Nikaido, acting as their protective caretaker while ensuring they stayed out of trouble. When he was sixteen years old, a devil approached him to take the Devil Exam, recognizing his unique and powerful magical abilities, and he left his family to pursue this path.

After successfully becoming a devil, Kawajiri existed for years in this elevated form, taking the name Asu. During this time, he observed Nikaido from afar and learned about the nature of her magic using his devil powers. He remembered that Nikaido had once saved his life when he was in crisis during the devil test, and he felt compelled to repay her kindness. Despite becoming a devil, Kawajiri retained a sense of gratitude and loyalty toward his adoptive sister, demonstrating that his fundamental concern for her welfare persisted even after his transformation and the passage of time.

Kawajiri first appears in the story as a devil named Asu, intervening to protect Nikaido from a dangerous contract with En, the powerful sorcerer boss. To delay the contract, he transformed Chota into Nikaido's form and teleported both Kaiman and Nikaido far away from En's reach. However, this interference incurred the wrath of Chidaruma, the most powerful naturally born devil, and as punishment, Kawajiri was demoted from his position as a devil back into a wizard. This demotion represents a dramatic fall from grace and fundamentally changes his circumstances and appearance throughout the remainder of the narrative.

After being demoted, Kawajiri reverts to the physical appearance he had when he was sixteen years old, just before his initial transformation into a devil. Although his actual age is thirty-two, his body now appears as that of a young adolescent, leading others to sometimes dismiss him as a cocky little boy despite his actual seniority and experience. His bare face is quite distinctive, with darkened skin around his eyes resembling panda marks or bruises, and a black nose that gives him a somewhat koala-like appearance. The shock of being demoted weighs heavily on his mind, along with practical concerns about his future livelihood and whether he will ever be able to grow into an adult form again.

As a wizard, Kawajiri possesses two related magical abilities: teleportation and clairvoyance. His magic allows him to jump into gaps in space to move to another location while simultaneously observing his destination through clairvoyant vision. As an elite wizard who passed the devil test at sixteen, he commands considerable magical power, and his abilities were potent enough to allow him to freely enter and exit En's headquarters as if it were his own home. Despite his demotion, he retains extensive knowledge about magic and demons, as well as a fearless attitude that stems from his former existence as a devil.

Kawajiri's personality combines remnants of his original caring nature with the strange eccentricities acquired during his time as a devil. Originally, he seems to have been kind and supportive toward his younger siblings, particularly Nikaido. However, his time as a demon has altered his character significantly. Compared to other devils, he is relatively sane, but he remains quite peculiar by human standards. He possesses considerable pride, frequently boasting about having passed the devil exam, and becomes extremely distressed when his abilities fail him, screaming to Nikaido that he does not want to see himself becoming useless as an elite wizard. He is also somewhat self-reflective, offering to provide counseling to others about their changed personalities, though he is himself consumed by unnecessary worries.

The relationship between Kawajiri and Nikaido forms the emotional core of his character arc. He has always wanted to help her, driven by gratitude for her past assistance and by the bond they shared as family when she was young. When they reunite in the sorcerer's world, he risks everything including his status as a devil to save her from her contract with En. After his demotion, he trains Nikaido in the use of her magic, explaining that she cannot change the past because she has not completely mastered her abilities. When Nikaido mutters oppa, an affectionate term for older brother, in her sleep, he wonders if she knows his real name and listens intently to her story about their shared past.

Kawajiri also develops an interesting dynamic with the character Reese, whom he initially dismisses as a brat. When Kawajiri loses his teleportation ability and becomes blind and unable to use his clairvoyance during a critical battle, he becomes extremely frustrated while watching Reese become active instead. Despite his earlier dismissiveness, he offers Reese crucial advice about the nature of Curse's Wrath, which ultimately helps Reese escape from a dangerous enemy. When he is captured, he sends Reese away, insisting that the younger wizard should finish the mission rather than try to rescue him, showing a pragmatic and self-sacrificing side beneath his pride.

Throughout the story, Kawajiri demonstrates remarkable resilience and resourcefulness. Even when strangled by a hose and unable to move, he refuses to accept defeat, struggling and declaring that someone who was a devil until recently cannot face such an end. He later uses his teleportation ability to help Nikaido and Kaiman escape from danger and escape to safety. After being saved and restored, he shows flashes of his old devilish personality, performing a strange mocking dance as revenge against those who had wronged him. In the epilogue, he decides to take the devil test again alongside Reese, indicating his enduring ambition and his refusal to accept permanent defeat.