John, also rarely known by his full name John O Guardian, is a male three-banded armadillo who serves as a familiar to the vampire Draluc. Approximately the size of a soccer ball, John has a distinctive appearance with white fluffy fur on his front and orange, highly durable armor on his back that allows him to curl into a protective sphere or launch himself as a projectile. Despite being unable to speak human language, he understands it perfectly and communicates using a personal vocabulary composed of the syllable nyu, as well as various gestures and signs. Only Draluc can fully understand his vocalizations.

John possesses a kind-hearted and fiercely loyal personality, constantly concerned about the well-being of his master. He becomes genuinely upset and has been known to run away from home if Draluc ignores him for too long, demonstrating an emotional sensitivity that underscores their deep bond. He has a notable sweet tooth and enjoys treats such as pancakes and fried chicken, while he strongly dislikes whole Chinese cabbage, stemming from a traumatic encounter with a vampirized cabbage that once drained his blood.

The background of John begins when he was a baby, separated from his family due to a vampire mosquito. He was discovered in an injured state by Draluc's grandfather, who gave the young armadillo to Draluc to raise. John's role in the story is that of a faithful companion and protector. On one occasion, he encountered other armadillos attacked by a vampire mosquito and tried to save them, finding the courage to defeat the creature after witnessing Draluc temporarily die in the struggle. This act led to John being hailed as a hero among armadillos, and Draluc initially decided to leave him behind with his own kind, believing it was for the best.

However, John became determined to reunite with Draluc and his companions after learning they had departed, using a photograph of the castle as his guide. His journey took him across great distances, receiving help from many people along the way, until he ultimately rescued Draluc from an attacking ostrich vampire. Recognizing the strength of their connection, Draluc officially made John his familiar by having him drink his blood, a decision that granted John immortality and solidified their partnership permanently.

Key relationships define much of John's character. His bond with Draluc is the central pillar of his existence, with the two frequently engaging in activities like hide-and-seek, bowling, and creative cooking. A running gag involves Draluc using John as a living shield to protect himself from dangers, which often results in John receiving the brunt of the impact. Despite this, their contract is unusually strong compared to modern familiar relationships, as it does not allow the master to unilaterally terminate the bond. Draluc also shows care for John's well-being in quieter moments, such as using moisturizing cream on his shell. John also maintains a strong connection with the armadillo community in South America, including his parents whom Draluc promised to look after.

Regarding abilities, John's primary power is the immortality he gained after officially becoming Draluc's familiar. His natural armadillo physiology allows him to curl into a tight ball, using his super-protective armor as defense or as an offensive rolling attack. His small, cute appearance also functions as a notable asset, making him instantly popular and beloved as a mascot figure to the extent that the entire town of Shin-Yokohama would mobilize to search for him if he were believed to be in danger. John has also demonstrated a particular skill at video games, being the only armadillo capable of scoring four to six against Draluc in their gaming sessions.