Tōma Kamijō is a high school student in Academy City, officially classified as a Level 0 esper incapable of measurable psychic phenomena. His defining trait is the "Imagine Breaker" ability within his right hand, which negates all supernatural powers—including magic, esper abilities, and divine blessings—upon contact. This power nullifies his own luck, causing chronic misfortune he frequently laments with the phrase "Such misfortune!" He views this misfortune as a necessary trade-off enabling him to perceive and aid those suffering around him. Imagine Breaker predates him as a metaphysical reference point for the world's stability and manifests as dragon-like entities with distinct supernatural properties—such as psychological manipulation, elemental control, or life-draining—when severed.

Born outside Academy City, Tōma endured childhood ostracization due to superstitions labeling him a "god of pestilence." His father enrolled him in Academy City to escape prejudice, though his misfortune persisted. Letters to his father indicated ongoing social challenges despite the move. Early in Academy City, he discovered Imagine Breaker's power, attracting attention from figures like Aleister Crowley. Classmates regard him as a luck-absorbing figure and resent his unintentional popularity with girls, humorously termed "Kami-yan disease" by friend Motoharu Tsuchimikado. Before the series' events, Tōma prevented suicide attempts by characters like Mitsuari Ayu and Shokuhou Misaki, losing memories of the latter due to anterograde amnesia.

Tōma exhibits extreme selflessness and impulsiveness, instinctively intervening to protect anyone in distress regardless of personal risk, often resulting in severe injuries. His actions stem from an innate drive to "do what is right in the moment," placing him above conventional notions of good and evil. He forgives former enemies and allies with them post-conflict, exemplified by relationships with Accelerator and Shiage Hamazura. After losing pre-series memories during Index's rescue, he adopted a more cautious demeanor to conceal his amnesia but retained core altruism. He shows no romantic awareness despite attracting female admirers, casually expressing a preference for older women like Kaori Kanzaki.

His development includes grappling with betrayal during the Hawaiian Invasion Arc, eroding his trust. In World War III, he confronted Fiamma of the Right and sacrificed himself to stop the archangel Gabriel, temporarily dying before reviving. This event deepened his introspection about righteousness and consequences of lying to Index about his amnesia. Later, in the "Omega World" arc, Othinus subjected him to a vision of a perfect reality without him, driving him to near-suicidal despair until the Will of the Misaka Network intervened. Rejecting this illusion, he chose to selfishly preserve his existence, symbolizing a shift toward self-affirmation. Throughout his journey, he inspired allies and foes toward redemption, becoming a unifying force in conflicts between science and magic factions.

Tōma's combat style relies on resourcefulness over formal training. He combines Imagine Breaker's negation with tactical improvisation, exploiting its limitations—like inability to nullify sustained or overwhelming powers—to disrupt adversaries. He possesses an innate danger sense ("Premonition Perception") alerting him to imminent threats. His physical resilience lets him endure incapacitating injuries, though vulnerable to conventional weapons like guns and blades. He wears Academy City's standard uniform with an orange T-shirt underneath and sports spiky black hair styled with gel, a trait sometimes mistaken for delinquency.