During the American occupation of Japan a young Magus from a fairly young family of homunculus crafters learned of the Third Grail War and seduced a nun that worked at the Church and got her to steal a fragment of the Lesser Grail. He takes the fragment and his new bride home, and nothing really comes of it until 1978 when he passes away and tells his son, Joseph Wilson, about it in his will. Joseph dreams of using the Grail to rocket his relatively young family to fame and fortune. Contacting several illustrious American Magus Families, he puts together a war and for a meager 3 million dollar entrance fee he offers them a chance to win the Holy Grail. The First American Grail War ends in colossal failure as Joseph planned on back stabbing the other participants from the very beginning and when he dies at the hands of Saber’s Master, the location of the Lesser Grail is lost with him. Ten years later the families come together again to try and succeed where they failed before.
Lawrence Rogers, originally Lawrence O’Keeffe is a fairly weak willed young man whose parents worked for the Rogers’ family estate. A relatively recent magus family, the O’Keeffe’s acted as the household servants for the Rogers’ for two generations. When Martin Rogers died in the 1979 Grail War his mother, Donna Rogers, the matriarch of the Rogers magus line, was devastated, both by the death of her son and by the prospect that her family line might go extinct. Knowing that seven year old Lawrence was the second son of the O’Keeffe family, Donna offered to buy Lawrence from the O’Keeffe’s. Offered several hundred thousand dollars for Lawrence, they accepted. Lawrence grew up under to tutelage of Donna, who never tired of deriding him and comparing him to her dead son. The bitter woman grew increasingly unsound as time went on, and began to flirt with Lawrence and tease him sexually. Their relationship never progress to anything physical, but it very much affected the way Lawrence behaved around women. He was with Donna when she summoned Caster as her Servant. Caster casually murders Donna in front of Lawrence moments after being summoned and asks him if he’d like to be her Master. Being a powerful and sexually manipulative woman, Caster is easily equipped to take control of the Master-Servant relationship and is a domineering psuedo-mother/lover figure to Lawrence. Going beyond Donna’s teasing, Caster actually uses sex as a tool to control Lawrence.
Harold Smith is one of two Master participating in the Second American Grail War that were not connected to the First. A forty-five year old Texan, Harold comes from a family magus line that’s only about 150 years old, but has a long history of military service. He himself served in Vietnam in a quasi-suicidal attempt to die gloriously on the battlefield. A lifelong smoker, Harold was recently diagnosed with stage four lung cancer. The cancer had spread throughout his body, to the point that not even magecraft could save him. Harold has no pretensions of actually winning the war and using the Grail to save himself. Instead, his main motivation is to die as most of the men in his family have, on the battlefield. He summons Bendigeidfran as Berserker. Berserker's very existence causes him nothing but pain, but Berserker’s NP, Pair Dadeni, keeps Harold alive despite his rapidly decaying body.
Kurome Wilson is the daughter of Joseph Wilson, the man that orchestrated the First American Grail War. She is a piss poor magus given that her father died when she was very young and also because she has utterly shunned magecraft since then. Before her father died he burned a summoning circle onto her left shoulder so that she would finish what he started and bring their family to greatness. She goes to college in Seattle, Washington and is surprised when she receives a package from her father containing a broken sword hilt as she is leaving her apartment. Walking around campus later that evening, she witnesses the clash between Berserker and Lancer. As she is about to be killed by Berserker, she accidentally summons Charlemagne as Saber. Charlemagne, motivated similarly to the No-name assassin in Strange Fake, wants to destroy the Grail as he sees its existence as a blasphemous twisting of the chalice that held the blood of Christ. Kurome has zero desire for the Grail as it caused nothing but suffering for her the last time it appeared. She doesn't care about her father's wish, she doesn’t even want to be Saber’s Master, but she also doesn’t want to die. So she reluctantly enters the Grail War.
Thomas Holdsworth is the only Master that survived the First American Grail War. He was hired by the Ultenburg family to participate in the war on their behalf. The Ultenburg family is an ancient linage that originally hailed from Austria, but for the past 200 years has resided in the Ipswich, Massachusetts. Though originally a great and powerful family, in the past century their magic circuits have grown increasingly weak. As with most of the other participants, the Ultenburgs don't care about the Root, they want the Grail to restore their failing Magic Circuits and save their line from extinction. Mr. Holdsworth was the last Master standing in the previous Grail War and was the Master of that war's Saber, in this war he has been relegated to being Assassin's Master. Because of the war's twisted nature because it is using a replica of the contaminated Grail, Servants that normally wouldn't be summoned can be called forth. As a result, when Holdsworth attempts to summon Nero as Saber, he instead summons the famous Roman poisoner that Nero saved from execution, Locasta, as Assassin. The two clash personality wise because he is a very dispassionate person and doesn't relish killing. Dealing in death is a job that is not to be enjoyed but to be endured. In contrast Locasta is a sadist and a sociopath and to which murder is practically a sacrament. They also clash because Assassin actively tries to make Holdsworth believe that he is not different from her and that is nothing more than a tool.
Rider's Master is 24 year old Donald Walker. Donald was spoiled endlessly by his mother after his father died in the previous war and acts like a typical whinny, spoiled, rich kid. He is the only Master that actually cares about finding the Root. Using an old Roman coin to summon his Servant, he is surprised, and mortified, to discover that his Servant is an elderly blind Roman soldier that somehow is Rider. Rider is devoted to him, but is seemingly worthless as a Servant and Donald is incredibly rude and unkind to the old man, believing him to be less than worthless. This all changes when they are attacked in the Seattle Mall by Locasta. Harold uses a command spell to accidentally active Rider's Noble Phantasm and the blind old beggar takes on his true form. The Rider and his incompetent Master slowly develop a friendship as Harold becomes someone worthy of Rider's devotion. Caster and Rider have a strong connection to one another, because Caster was Rider’s Empress in life. Caster forced him to compromise his values and destroyed his life despite Rider’s lifelong loyalty to her husband. As a result Caster and Rider have a unique relationship in the war and loath one another.
Martin Forester is a 67 year old magus whose reason for participating in the war is to try and use the miracle to resurrect his son, Kyle. Kyle represented their family in the last war and was the first Master to die. Martin feels personally responsible for his son's death because he pressured him to enter the war. Kyle's death has haunted Martin ever since, and shredded his relationship with his daughter-in-law and his grandchildren. He summons Archer.
I haven’t really thought much about Lancer’s Master since Lancer gets offed by Berserker in her first appearance. I was kind of thinking about having Lancer’s Master be a younger Tokiomi Tohsaka. After the death of his Servant, Tokiomi refuses to withdraw from the War and continues to try and participate to the best of his abilities, later making a contract with Archer after his Master dies. Lancer’s identity would be Boudicca not that that really matters given that Berserker rips her in half.