John is the third son of his father, a mage who hide his magical status from most of his family, and his mother, who worked for the police. At a very young age, John saw his mother being incinerated to death, while on duty, when it was shown on TV and his father, who was nearby, quickly turned off the television to shield his son from seeing the harrowing scene. While mostly percieved as a phlegmatic individual, John's father was a loving parent that continued to take care of his family as he told a young John about his mother's achievements in order to bring solace to his child's mind. John later joined Orlando Reeve's Clan Calatin which his benefited father, who was estranged from the Mages Association, as he would receive the American government’s financial backing. After learning the existence of magecraft, John was horrified when learning how useful magecraft could be for crime.
In preparation to help Orlando Reeve win the Holy Grail War, False Caster equips Clan Calatin with Noble Phantasms he created. John is armed with a dagger.
When False Assassin attacks the police station, John and the other Clan Calatin members try to fight her off, doing rather well, but she kills several of them. Suddenly, her Master Jester Karture arrives and teleports her away with a Command Spell. Jester takes over the battle and kills several of them, and cuts off John's right hand when he tries to stab him. They are saved when Hansa Cervantes intervenes and battles Jester, eventually forcing him to retreat.
False Caster gives Clan Calatin new Noble Phantasms and gives John an artifical hand that has a built-in dagger coated in Hydra venom. False Caster takes a liking to John, recites his history, and says he has potential. The officers ask if they actually stand a chance in the war. False Caster says he is not sure, but they can make a difference.
Orlando assigns the officers to secure Tsubaki Kuruoka, a young comatose girl who is the Master of False Rider. When they arrive at the hospital, True Archer arrives, intending to kill Tsubaki. He summons the three-headed hellhound Cerberus with his Noble Phantasm, King's Order, to attack the officers and they find themselves outmatched. Around this time, False Caster decides to use his Noble Phantasm, Musketeers' Masquerade, on John to turn him into a true hero. False Berserker arrives and defeats Cerberus, but is defeated in turn by True Archer.
False Archer, Saber, False Assassin, and Sigma arrive, but False Archer becomes offended by Saber and attacks him. After observing their battle for a bit, True Archer turns to the officers and says he has no quarrel with them, so if they will stand aside and let him murder Tsubaki, he will spare their lives. John steps foward and says he will fight to the end to defend the child. True Archer says he is brave, and for that, he pities him. He strikes John with his bow, launching him into a building and breaking his neck. John expects himself to die, but mysteriously feels no pain and is not losing consciousness. He is then surprised when he sees a vision of a time when False Caster in his original life as Alexandre Dumas met Edmond Dantès.
John is healed and empowered by Musketeers' Masquerade. He rejoins the battle, to the shock of his fellow officers and True Archer, then knocks True Archer back with a flying knee. He attacks True Archer with punches and kicks, and his strength is now great enough to hurt him. He then draws his Hydra venom dagger, which horrifies True Archer and makes him jump back. John tells the other officers to secure Tsubaki while he keeps True Archer busy. True Archer draws his bow and declares John a worthy opponent.
While John continues to battle True Archer, his fellow officers try to assist him, only to be scattered by arrows. False Berserker reappears and attacks True Archer from behind in the form of a squad of officers, but True Archer slashes at them without even turning around, keeping his attention on John. False Berserker attacks again and manages to bind True Archer with hands from shadows.
True Archer spots False Berserker's Master Flat Escardos and seemingly kills him, but he turns out to be a duplicate while the real Flat was hidden in the squad and uses a spell combined with his second Command Spell to disrupt the Servant's flow of magical energy by targeting the From Hell he stole, nearly causing his body to fall apart. John then stabs him in the side with the dagger so hard the dagger breaks. As True Archer's body is slowly eaten away by the venom, he asks False Berserker why he risked his Master on the battlefield, so they explain how they needed that second Command Spell for their plan to work. True Archer congratulates all three of them for doing all this damage to him without divine aid, but then says it is not enough. To their horror, Black Mud erupts from his wound and absorbs the venom before flowing back inside him and healing the wound. A tornado of magical energy erupts from him, knocking them back. He suddenly decides they are beneath his notice and leaps away to attack False Archer instead. Later, a wave of blackness washes over the street. When it passes, John and all the other combatants have vanished, to the shock of those observing.
John sees another vision of the time Alexandre Dumas met Edmond Dantes. He wakes up next to his fellow officers, who confirm they saw the same thing. His artifical hand is damaged. They ask him how he became so powerful and he says he is not sure, but he thinks False Caster did something to him. They look around and are shocked because they seem to be in the same place they were before, but the streets are empty and undamaged as if there was no battle.
They explore the city and find a few people whose minds have been seized by something, and discover if they try to walk out of the city, they are warped back into it. They come to the church and find Ayaka Sajyou and her Saber. His fellow officer Vera Levitt does the talking to invite them to work together. They conclude they are in an isolated space, possibly a Reality Marble, and they must eliminate whoever is causing it to escape.
John has knowledge of Magecraft and decent skill in hand to hand combat and weaponry. After losing his right hand to Jester Karture, False Caster gives him a prosthetic hand that has a built in dagger coated in Hydra venom. After being healed and empowered by Musketeers' Masquerade, his strength, speed, and agility increase dramatically. He becomes able to leap high enough to strike True Archer who was flying in the air, and strong and fast enough to harm him with punches and kicks while dodging his attacks.