Alexander Anderson is a major character in Hellsing, serving as a Catholic priest and the primary field agent for the Vatican's Section XIII, an organization known as Iscariot which is dedicated to eliminating vampires and other supernatural threats to the Church. He is a tall, lean man with short, spiky blond hair, green eyes, a defined square jaw, and a prominent wedge-shaped scar on his left cheek. He typically wears round glasses, a large cassock, white gloves, and a cross necklace. His gloves bear inscriptions: on the right hand, Jesus Christ is in Heaven, and on the left, Speak with the Dead. Anderson is often known by various monikers, including the Regenerator, Bayonet Anderson, and the Angel Dust.

Personality wise, Anderson is a figure of stark duality. In his daily life, he appears as a calm and collected priest who runs a Catholic orphanage near Rome, showing genuine kindness and gentleness towards the children under his care. However, this peaceful demeanor masks a deeply fanatical and obsessive nature. He is an ardent Catholic fundamentalist who views his work as a holy crusade against all heretics, non-believers, and monsters. In battle, he is merciless, relentless, and almost unstoppable, often compulsively quoting passages from the Bible as he engages his enemies. He holds an intense hatred for Protestants, referring to Sir Integra Hellsing as Babylon, though he ultimately demonstrates a sense of honor and is disgusted by the senseless slaughter of innocent civilians, even if they are non-Catholics.

Anderson's primary motivation is the absolute eradication of all threats to the Catholic Church, which he equates with the will of God. His driving obsession is to destroy Alucard, the Hellsing Organization's most powerful vampire, whom he sees as the ultimate abomination and heretic. He is willing to achieve this goal at any cost, even if it means sacrificing his own humanity or life itself.

Within the story, Anderson serves as a rival and foil to Alucard. He is an antagonist who often finds himself at odds with the Hellsing Organization, but their mutual enemy, the Millennium Nazi vampire army, sometimes forces a temporary and uneasy alignment of interests. His role evolves from a simple executioner to a key player in the London battle, where he confronts the rogue Archbishop Enrico Maxwell for his genocidal actions before turning his full attention to his final duel with Alucard.

Key relationships define his character arc. His most significant relationship is with Alucard, whom he respects as a worthy opponent, creating a complex dynamic of mutual animosity and grudging admiration. He is the mentor figure to Enrico Maxwell, the head of Iscariot, and is deeply saddened when Maxwell succumbs to his lust for power and begins slaughtering innocents. He also acts as a father figure to other Iscariot agents like Heinkel Wolfe and Yumie Takagi, having raised them in the orphanage. He shows a begrudging respect for Integra Hellsing's conviction and for Seras Victoria's bravery, even acknowledging when Seras has surpassed the abilities of his ordinary soldiers.

Anderson undergoes a significant but tragic development. Initially presented as a purely fanatical killing machine in the first television anime, his character in the manga and OVA is more complex, demonstrating moral depth by rejecting Maxwell's orders to kill innocents and instead choosing to escort Integra to safety. His ultimate development is his final, desperate transformation. Realizing he cannot kill Alucard as a human, he chooses to impale his own heart with the Nail of Helena, a holy relic from the True Cross. This act transforms him into a monstrous, plant-like creature of immense holy power. Alucard, who had hoped to be killed by the human Anderson, is deeply disappointed and enraged by this choice, declaring that Anderson has forfeited his humanity and must be destroyed.

Anderson possesses a wide array of notable abilities. He is a regenerator, capable of healing almost any wound, including gunshots to the head, due to advanced biotechnology, though he attributes this power to a divine gift from God. His primary weapons are hundreds of blessed silver bayonets, which he wields with superhuman strength and speed, allowing him to cut through steel and overwhelm vampires. He can use pages from his Bible to create holy barriers that trap his enemies or to teleport himself across distances. In his final transformed state using the Nail of Helena, his power increases exponentially; he gains control over thorny vines, his bayonets become wreathed in holy fire capable of burning Alucard's soul army, and his regenerative abilities become nearly absolute. This form ultimately proves to be his undoing, as Alucard reaches into his chest and rips out the nail along with his heart, causing Anderson to turn to ash.