John is a Shadow, a noble, soot-bodied being who resides in the mysterious Shadows House, and his counterpart and caretaker is his Living Doll, Shaun. As a Shadow, John is entirely black and appears as a silhouette of Shaun, sharing his build and facial features. He is depicted as a young boy with short, unkempt hair and typically wears light blue Victorian-style clothing, including a long coat with frills and white socks. Shaun, in contrast, is his human "face," with pale skin, dark hair, and dark indigo eyes, who wears the standard uniform of a servant.

Their personalities stand in stark contrast to one another, forming a complementary pair. John is a boisterous, muscle-headed extrovert known for his friendly and open-minded nature. He acts on impulse, often charging headfirst into situations without a second thought. Shaun is his opposite: a quiet, thoughtful, and observant introvert. He is hardworking, intelligent, and serves as the voice of reason, often having to curb John's more reckless tendencies. Despite these differences, or perhaps because of them, the two share an exceptionally strong bond. John defies the rigid norms of the Shadows House, which dictate that a Living Doll should lose their own personality to perfectly mirror their Shadow master. Instead, he treats Shaun with immense respect and genuine care. A notable example of this is when John offered to wear glasses himself so that Shaun, who has poor eyesight, could wear his own without raising suspicion, an act of solidarity that is highly unusual for a Shadow.

John is driven by straightforward and loyal motivations. He harbors a persistent, one-sided romantic affection for Kate, another Shadow, having fallen for her at first sight. This leads him to repeatedly propose marriage to her, declaring he will become the king of the Shadows House, much to her annoyance. However, beneath his comedic devotion lies a deep trust in Kate's judgment. When she reveals the sinister truths about the house, including the brainwashing of Living Dolls, John becomes a committed and reliable ally in her covert efforts to understand and oppose the establishment. Within the story, John acts as a key supporting protagonist. His immense physical power and unwavering loyalty make him crucial during action sequences and risky operations, such as the plan to infiltrate the Adults' Building. He often serves as a catalyst for action, pushing the group forward with his enthusiasm and strength.

The most significant relationship is, of course, with his own face, Shaun. Their partnership evolves from a standard master-servant dynamic into one of mutual reliance and friendship. They share small rituals, such as a goodnight kiss, and communicate with a deep understanding of one another. When Shaun is affected by the brainwashing soot coffee, John instinctively knows how to save him, not with kindness, but through a violent fistfight that forces Shaun to revert to his true self, demonstrating a unique and profound connection. John is also cordial with other inhabitants. He thinks of Kate's Living Doll, Emilico, as a little sister, as their energetic personalities are quite similar. He often clashes with the haughty Shadow Patrick, but their rivalry is not born of malice and is often depicted in a comedic, bickering light.

John possesses an extraordinarily high quantity of soot, the magical substance generated by Shadows. His power is a physical type, allowing him to channel this soot into his fists to create explosive concussive blasts. He has named his signature attack the John Punch, which he first demonstrated by destroying a massive rolling boulder with a single hit. The primary limitation of this ability is its range; its effectiveness drops drastically beyond two or three meters. He later develops a defensive application of this power, called the John Shield. When he generates soot, it often manifests visibly, resembling colorful fireworks that reflect his intense emotions.