Allelujah Haptism is a central character whose life is defined by the consequences of human experimentation and his struggle for identity. Born on February 27, 2288, his early history is shrouded in mystery, as he was orphaned as a child in Kazakhstan before being kidnapped and taken to the Human Reform League's Super Human Research Institute. There, he was designated as Subject E-0057 and subjected to procedures designed to create a brainwave-altered super soldier. The experiments granted him enhanced physical abilities and quantum brainwaves but also fractured his psyche, creating a violent alternate personality known as Hallelujah. During his time in the facility, he formed a profound connection with a girl named Marie Parfacy, who gave him the name Allelujah, meaning a word of praise to God, as an expression of gratitude for their existence. After being deemed a failed experiment and marked for disposal, he escaped with other test subjects in a space shuttle. As resources dwindled and survival became uncertain, the Hallelujah personality emerged and killed the others to ensure his own survival, an act that would haunt Allelujah forever.

In terms of personality, Allelujah is primarily a quiet, gentle, and introspective individual who deeply values life and shows great concern for those close to him. He is characterized by a profound guilt regarding his role as a soldier and the act of killing, often struggling to reconcile his mission with his conscience. He hates war but feels his fate as a super soldier has destined him for the battlefield, a contradiction he accepts by serving with the private armed organization Celestial Being in the hope of ending global conflict. In stark contrast, his alternate personality, Hallelujah, is murderous, ruthless, and finds amusement in the suffering of others. Hallelujah operates purely on a survival instinct, willing to do anything necessary to protect himself and Allelujah, often criticizing the primary personality's hesitation as a liability. Visually, these two states are distinguished by which eye is uncovered; Allelujah typically reveals his left gray eye, while Hallelujah reveals his right gold eye. The two often converse internally, representing a constant debate between morality and pragmatism.

Motivationally, Allelujah is driven by the desire to prevent others from suffering the same fate he did. He fights to eradicate the systems that create super soldiers and to ensure there will be no more victims like himself or Marie Parfacy. This mission is deeply personal, and his primary goal becomes intertwined with rescuing and protecting Marie, who operates as the enemy pilot Soma Peries. His role in the story is as a Gundam Meister for Celestial Being, piloting transformable mobile suits that match his need for high-speed mobility and reflexes, beginning with the Gundam Kyrios in the first season and later the Gundam Arios in the second season. He is a key member of the crew aboard the spaceship Ptolemaios, working alongside other Meisters to carry out armed interventions worldwide.

Key relationships define his arc. The most critical is with Marie Parfacy, also known as Soma Peries. As children in the research facility, they shared a deep emotional bond through their quantum brainwaves. In the present, their identical brainwave frequencies cause them intense physical pain when near each other, forcing them into conflict as enemies. Allelujah's journey is largely about breaking through that conflict to reach the person he knew as a child. His relationship with his other self, Hallelujah, is equally central; it is a symbiotic and contentious partnership, with Allelujah eventually learning to accept and integrate Hallelujah's combat instincts to become a true super soldier. Within Celestial Being, he shares a professional bond with his fellow Meisters, including Setsuna F. Seiei and Lockon Stratos, and respects the tactical decisions of the crew's commander, Sumeragi Lee Noriega.

Allelujah undergoes significant development throughout the narrative. Initially, he is paralyzed by guilt and reliant on Hallelujah to survive deadly situations. A major turning point occurs when he chooses to embrace Hallelujah's power as his own, allowing the two personalities to fuse their thinking and reflexes, unleashing his full potential as a super soldier. Following the first season's final battle, he is captured and imprisoned for four years, during which his quantum brainwaves are damaged, and he loses Hallelujah. Rescued in the second season, he must learn to fight as a normal soldier while rekindling his connection with Marie. Through exposure to high concentrations of GN Particles, his abilities and Hallelujah gradually return. By the end of the second season, he achieves a greater sense of inner peace, accepts his past, and leaves Celestial Being with Marie to find a new purpose in life beyond constant warfare. In the follow-up story Re:(in)novation, which takes place after the conflict with the Extraterrestrial Living-metal Shapeshifters, Allelujah remains active with Celestial Being, working alongside his comrades to confront new threats from a human supremacist organization called Humarest, showing his continued commitment to protecting peace.

Notable abilities stem from his status as a genetically engineered super soldier. He possesses quantum brainwaves that grant him superhuman reflexes, spatial awareness, and the ability to sense other brainwave users. His body has been physically modified for peak combat performance, making him stronger and faster than a normal human. In his integrated state, where Allelujah's strategic thinking combines with Hallelujah's battle intuition and killer instinct, he becomes an exceptionally formidable pilot capable of processing information and reacting at incredible speeds. This makes him uniquely suited for piloting high-speed, transformable mobile suits in atmospheric and space combat.