Halloween, whose original name was Pump, is a major antagonist in the MÄR series as a high-ranking member of the Chess Pieces, holding the position of Knight and being recognized as the second strongest among them. His past is marked by tragedy and abandonment; as a child, he was left by his parents and subsequently bullied by other children in his unnamed hometown. During this time, he was befriended by another boy named Alan, who would often rescue him when he climbed trees to escape his tormentors. His childhood behavior included a disturbing penchant for dismembering live animals, a habit Alan tried to discourage. The pivotal moment in his youth came when Alan showed him a flame-producing Nature ÄRM and promised to give it to him once he awakened his own magical powers. Unable to wait, Pump stole the ÄRM and fled into the woods. After driving away a pack of animals with the stolen item, he was found by Alan, but refused to return and used the flames to block his friend's path. The following day, all the children who had bullied him were found murdered, making Pump the prime suspect, after which he vanished from the town.

Halloween’s personality is defined by profound sadism and a genuine love for war and bloodshed. He takes immense pleasure in the pain and suffering of others, whether they are enemies, lower-ranking Chess Pieces, or even his fellow Knights. He often threatened the Knight Rapunzel with death to curb her greed and took joy in mentally tormenting the Rook Ian by informing him of his lover Gido’s punishment. Despite his cruel exterior, Alan points out that deep inside, Halloween remains the same cowardly child he once was. His hatred for Alan is the central pillar of his emotional life. He is thrilled by battles with him, relishing the chance to hurt the only person who once opposed him. His perspective is twisted; he genuinely believes Alan was mocking him during their childhood, which transformed him into the perfect model of a Chess Piece’s psychology. Alan calling him by his childhood name, Pump, became a major taboo, and his mask, which resembles a tomato, earned him the nickname Tomato Head from his rival.

In the story, Halloween is often entrusted with leading major Chess Piece operations, such as the invasion and massacre at Acalupa Port, which ended with the city destroyed and most of its population murdered. He is also responsible for punishing members who fail their missions. He had the Knight Chimera torture Gido and then taunted Ian about it, sadistically wishing Ian were strong enough to kill him. His most significant and personal role is his deep-seated rivalry with Alan. Prior to the main events, Halloween fought Alan during the final round of the first War Games; the battle ended in a draw, leaving both near death. To commemorate the fight, Halloween used his Darkness ÄRM, Igniel, to seal Alan inside the body of a dog named Edward, who served as the watchdog for Lestava’s royal family. This act set the stage for their final confrontation during the second War Games. In their climactic battle, Halloween was relentless, aiming attacks at innocent spectators to force Alan to shield them with his own body. He revealed his true identity as Pump to further torment Alan. Ultimately, Halloween unleashed his powerful Guardian ÄRM, Wakan Tanka, but was defeated when Alan summoned his own Guardian, Saint Anger, which crushed the spirit and sent Halloween flying into the sky to his unknown demise. In his final moments, he finally acknowledged Alan’s true strength.

Halloween possesses a wide and powerful array of ÄRMs. He is known for having chained a large cross to his back, which itself is a Weapon ÄRM called Cross Dagger that can transform and shoot missiles of flame. His other flame-based abilities include Napalm Death, a blossom on his cross that shoots flames like a cannon; Antares, which creates balls of fire; and Sticks of Fire, two flaming sticks held by the Flame Hand ÄRM that he uses to create hands for himself since he is bound to the cross. He also wields the stolen Nature ÄRM from his childhood that produces flames. His Darkness ÄRMs include Grave Hail, which in the anime summons coffins with eyes and mouths that open to release skeletal warriors to do his bidding. Another Darkness ÄRM, Igniel, is used to merge two living creatures together, which was how he fused Alan with the dog Edward. His Guardian ÄRM, Trick-Or-Treat, creates explosive plant creatures with jack-o-lantern heads. His most powerful Guardian is Wakan Tanka, a massive, winged, skeleton-bodied skull that breathes scorching flames, embodying a divine sacred spirit whose omnipotence is limited by being summoned through Halloween’s cross pendant.