The character known in the fandom as Demon Dog is officially named Loopy Toad within the narrative of The Daily Life of the Immortal King. This seemingly simple green dog has a deceptively grand origin, having lived a previous life as a powerful, self-aware demon king of the fifth rank. After its physical form as a toad was destroyed, the last remnant of its spirit managed to escape only to be summoned, fused with another spirit, and eventually reincarnated into the body of a dog in the human world. This forced reincarnation into a different species is a source of constant, low-level indignation for the creature, who remembers its former glory and power.

In its current canine form, Loopy Toad possesses a personality that is a complex mixture of world-weariness, secret pride, and begrudging acceptance. It is often seen with a dispirited expression, listlessly observing its surroundings as if mourning its lost status as a demon king. Despite its low-energy demeanor, it has not abandoned all ambition, as it secretly continues to cultivate and attempts to adapt to its new body. It displays a strong sense of dignity, which is frequently challenged by its new life. This is most evident when its classmates try to teach it to bark like an ordinary dog, a task it finds deeply humiliating. Its personality reveals a tsundere streak; it outwardly protests and condemns the loss of its dignity, but its new body's instincts eventually betray it, causing it to let out an involuntary "woof" when sufficiently provoked.

Its primary motivation shifts from a desire to escape and regain its former power to a more grudging acceptance of its new reality. While it initially dreams of escaping the confines of the school to find more appealing food like plump flies, these ambitions are quickly quashed by the immense power it senses in its new environment. Ultimately, its core motivation becomes simple survival and adaptation, as it understands that open rebellion is futile. It works to blend into communal life, learning to behave more like a dog and even allowing students to pinch its cheeks, all while secretly trying to build up its cultivation once more.

In the story, Loopy Toad serves as a recurring presence within the classroom and later becomes the pet of the protagonist, Wang Ling. It is a popular figure at No. 60 High School, with its green akita appearance drawing attention from students in other classes who bring it treats. However, its most significant role is in relation to Wang Ling. Unbeknownst to most, Loopy Toad is intimately familiar with Wang Ling's true power; six years prior, the ten-year-old Wang Ling destroyed its demon king form with a single punch. This history places Loopy Toad in a unique position as one of the few beings who understands just how terrifying Wang Ling is, making its obedience to him a matter of profound fear rather than simple domestication.

Its key relationships are primarily with its classmates and with Wang Ling. The students, particularly Hero Guo (known as Master of Dopey), take an active interest in training it. Hero Guo attempts to use videos and even a poster of the Heavenly Dog to inspire Loopy Toad, while the fujoshi classmate Feather Lin uses bizarre shipping logic to tease it into barking. Loopy Toad's relationship with Wang Ling is the most defining. It lives in constant terror of Wang Ling's "Killer Eye," a look that instantly evokes the deepest fear in its soul, forcing it into immediate submission. At the same time, it holds a grudging respect for Wang Ling, recognizing his use of legendary techniques like the Great Instant Transport Spell, which makes Loopy Toad feel that its own millennium of cultivation was insignificant.

Over the course of the narrative, Loopy Toad undergoes notable development. It evolves from a secretly scheming demon king hoping to escape and regain its power to a more compliant figure that reluctantly accepts its position as Wang Ling's pet. It learns to bark more proficiently, its posture when eating bones becomes more dog-like, and it generally shows signs of casting aside its former identity to integrate into its new life. However, this development is not a true change of heart but a pragmatic surrender to an overwhelming power it cannot challenge. Its final moments of inner monologue, where it facepalms at the thought of just obeying Wang Ling, highlight the complete annihilation of its former demon king integrity.

Regarding its abilities, in its current dog form, Loopy Toad does not display the immense power it once held. Its most notable ability is its sheer resilience, having survived the destruction of its physical form and successfully reincarnated, which is a testament to its strength as a former demon king. It retains its full self-awareness and memories of its past life, including knowledge of the Three Thousand Great Spells. It is also shown to be secretly attempting to cultivate again, suggesting that its potential for power remains, even if it is currently suppressed. Its primary defensive ability, however, is its fear-based obedience to Wang Ling, which has kept it safe and alive where its own pride would have led to its destruction.