Big Johnny is an alien creature discovered by the hero Ah Sheng within the wreckage of an alien spaceship. Fearing that the small being would be subjected to cruel experiments, Ah Sheng secretly brought it home. There, his young son, Little Johnny, remembering his mother's story about children falling from the sky as stars to choose their parents, immediately adopted the creature as his own son and named him Big Johnny.

In its standard, calm state, Big Johnny is a small, white-furred creature resembling a cat or a plush toy, standing only about thirty centimeters tall. It has a long tail, a unicorn-like horn on its forehead that fades from gold to pink, and distinctive hot pink markings on its eyes, ears, cheeks, and back. Despite this adorable appearance, Big Johnny possesses a powerful and destructive alternate form. When provoked by extreme negative emotions, particularly the rage or grief of Little Johnny, it transforms into a gigantic, building-sized canine-like creature. In this berserk state, its body turns mostly black, and its glowing markings shift from pink to a bright, menacing green.

Big Johnny's personality is largely childlike and mischievous. It has a huge appetite and will attempt to eat almost anything that captures its interest, from mop water and lightbulbs to the whisper flowers growing on a grave. It is energetic, prone to running off uncontrollably, and often causes trouble that requires Little Johnny to intervene. Despite this playful and simple demeanor, Big Johnny demonstrates signs of higher intelligence, such as recognizing the ruins of its native alien ship and investigating to see if others of its kind remain. It is also capable of holding grudges, shown by its hostile reactions upon meeting certain individuals like the hero Ghostblade.

The central pillar of Big Johnny's identity is its relationship with Little Johnny, whom it considers its father. This bond is reinforced by a psychic connection between the two. Big Johnny senses Little Johnny's emotions directly, and intense feelings of anger, grief, or even depression can trigger its involuntary and destructive transformation. Conversely, Little Johnny is the only one who can calm Big Johnny down and revert it to its harmless form. This volatile dynamic is a constant challenge, as Big Johnny is helpless against the emotional storms of its young parent. Big Johnny was also deeply bonded to Ah Sheng, viewing him as a grandfather, and was visibly grief-stricken alongside Little Johnny upon finding Ah Sheng's body.

In the story, Big Johnny is a co-protagonist of a major story arc that explores its past and its bond with Little Johnny. It also appears as a supporting character in other arcs, including those centered on the heroes Queen and Ghostblade. Its nature as an alien makes it a unique and dangerous entity within the world of heroes. Unlike humans, Big Johnny is not governed by the Trust System, granting it a significant resistance to abilities based on Trust or Fear. This makes it immune to powers like compression, mind-altering hallucinations, and boundary rules, allowing it to fight on equal or advantageous terms against powerful heroes who rely on such abilities. Its primary weakness is that while it is immune to mental attacks, its partner Little Johnny is not; an enemy can affect Big Johnny indirectly by targeting the boy through their psychic link. Big Johnny can also still be harmed by physical phenomena, such as electricity or telekinetically thrown objects.