Neil Marshall was the prosecutor in charge of the SL-9 case; it was the first time that he and his brother Jake worked together. On that day, Neil Marshall was named King of Prosecutors. He took a picture of himself holding his King of Prosecutors trophy with Detective Lana Skye and Chief Detective Damon Gant. After the award ceremony, Marshall and Gant went off to question Joe Darke, the suspect in the SL-9 Incident, about all the crimes he had committed.
A sudden thunderstorm shut off the power, allowing Darke to escape from the room. The prosecutor chased off after him and found him in the office of Detectives Gant and Skye, where Darke was holding Lana's younger sister Ema hostage. Marshall attempted to save her by knocking Darke unconscious with the award he had just received. Ema, believing the man to be Darke, shoved Marshall aside and fell unconscious. Gant came in afterward and, seeing an opportunity to solidify the case against Darke and therefore secure his promotion to chief of police, proceeded to lift Marshall's unconscious body and place it onto the spear of a suit of armour in the room, thereby killing the unfortunate prosecutor. Gant also modified the evidence to make it seem like Ema was the true killer of Marshall, in case anyone examined the case too closely. This allowed him to blackmail Lana into becoming Chief Prosecutor, thereby allowing him control not only over the police force, but over the prosecutor's office as well.
Neil's brother Jake always felt that something was amiss with the case, but could never find out what. It wasn't until Phoenix Wright was called to defend Lana against a murdercharge that the true killer of Neil Marshall was eventually revealed and arrested.
The kanji in his Japanese surname "Zaimon" (罪門) mean "sin" and "gate". The development team claim that it fits well with Gant's Japanese name.
His given name in the Japanese version, "Naoto" (直斗), roughly means "straight fight" and is meant to imply honesty.
"Marshall" is an English and Scottish surname. Originally it was meant as either a status surname for someone in charge of the horses belonging to a royal household, or as an occupational surname for someone who looked after horses, or was responsible for the custody of prisoners. This does fit with Neil Marshall's character, since he wears typical cowboy clothing and cowboys are usually associated with horses, and his job as a prosecutor involves detaining criminals. It is also similar to the word "marshal" and is pronounced identically.
"Neil Marshall" and "Jake Marshall" could also be plays on "real marshal" and "fake marshal" respectively.