Ace Attorney is a series of legal thriller comedy-drama adventure/visual novel games created by Shu Takumi. Players assume the role of a defense attorney in a fictional courtroom setting in the main series. Published by Capcom, the series includes Phoenix Wright: Ace Attorney, Phoenix Wright: Ace Attorney – Justice for All, Phoenix Wright: Ace Attorney – Trials and Tribulations, Apollo Justice: Ace Attorney, Ace Attorney Investigations: Miles Edgeworth, Ace Attorney Investigations 2: Prosecutor's Gambit, Professor Layton vs. Phoenix Wright: Ace Attorney, Phoenix Wright: Ace Attorney – Dual Destinies, The Great Ace Attorney: Adventures, Phoenix Wright: Ace Attorney – Spirit of Justice, and The Great Ace Attorney 2: Resolve. Character names for the English release of the series were changed significantly from the original Japanese release.

Phoenix Wright (成歩堂 龍一, Naruhodō Ryūichi) is a defense attorney and the main character of the franchise, and the protagonist in all games in the main series, except for Apollo Justice.

Mia Fey (綾里 千尋, Ayasato Chihiro) first appeared in Phoenix Wright: Ace Attorney as Phoenix's boss and mentor at Fey and Co. Law Offices, assisting him before she is murdered and her sister, Maya Fey, framed for the crime. Phoenix successfully defends Maya, and learns of both sisters having spirit channeling powers, with Maya managing to channel Mia. Mia is channeled through Maya and later their cousin Pearl Fey at various points during the series' story. In Phoenix Wright: Ace Attorney – Justice for All, Mia again assists Phoenix in proving Maya's innocence after their aunt, Morgan Fey, frames Maya for murder in order to have Maya replaced as the master of the spirit channeling technique by Pearl. Mia appears again in Phoenix Wright: Ace Attorney – Trials and Tribulations, where her backstory is explored, particularly her defending Phoenix for murder and proving a woman named Dahlia Hawthorne was the true culprit. It is later revealed that Dahlia was involved in her first trial, causing the death of her defendant and getting away with her crimes before also poisoning her lover, Diego Armando. She later assists Phoenix in defeating Dahlia after Dahlia attempts a plot to kill Maya.

Mia was created by the series' creator, Shu Takumi. Originally, Takumi planned to have Mia's law office consist of a protagonist, helper, and mentor, but this changed when Takumi's boss told him to remove one, arguing that three was too many. Takumi did not want to do this, choosing to handle it by having Mia die and exist as a spirit through a spirit medium, saying that this would end up being two characters. He stated that he considered having Mia be a spirit from the very beginning of the game, and positive reception during development spurred on this idea. The first trial was intended to be about the murder of Phoenix's mentor with Phoenix as the defendant, but this was scrapped with the idea being that the death wouldn't have an impact if the player had not met the mentor before. Thus, he introduced a new prologue with the mentor assisting Phoenix in his defense. The spirit medium angle was planned to be used only in the first game, but expanded this aspect in the second and third games, where the third entry became centered around spirit mediums.^{[citation needed]} Takumi expressed worry, when designing her younger self for Trials and Tribulations, about pulling it off properly. When designing the fourth episode of Trials and Tribulations, Takumi wanted Mia to face off against Edgeworth, but experienced complications due to both having not lost a case before. He ultimately decided to have defendant Terry Fawles die during court in order for the trial to end without a verdict in order to deal with this.

Mia was designed by Tatsurō Iwamoto, who originally conceived her as an older man wearing a geta. They decided to change this after determining that too many of the characters were old men, causing them to change Phoenix's mentor character to be a young woman. The original old man design was repurposed for the Judge's design. When designing Maya in Phoenix Wright: Ace Attorney – Spirit of Justice, the game's character artist, Takuro Fuse, considered making her resemble Mia now that she is grown up, ultimately coming up with a design that felt similar to Mia's while attempting to not lose Maya's charm. The theme song for Maya and Mia, "Turnabout Sisters Ballad," was originally written to be cheerier, but once Mia's death was written, the song was changed by adding a semitonal chord shift to the main part of the song to express sadness.

Mia Fey has received generally positive reception. Dengeki writer Ayana felt that her words to Phoenix, to reverse his way of thinking, resonated with her, feeling that being able to change one's way of thinking was a valuable quality in all walks of life. Eurogamer writer Jay Castello, while discussing their desire to become a lawyer as a teenager, stated that Mia was the character who drove this desire. They called her "compassionate, dedicated, and hyper-competent," and that she was the kind of person they aspired to be. Fellow Eurogamer writer John Walker called her a "wonderful" character, enjoying her presence in the game while expressing that he had a lot of affection for her as a female video game character. Game Revolution writer Joe Dodson discussed her sexuality, particularly noting that her breasts were particularly large and commenting on how weird it is that she can possess Pearl, an eight-year-old, giving her breasts. Nintendo Life writer Kate Gray also commented on Mia's design when channeled by Pearl, calling it creepy. On Mia's character, she felt that Mia's death was meant to teach players not to take anything for granted, comparing this scene to a later case where Phoenix's client turns out to be guilty. She felt that this twist was written with the idea that you'd grown complacent. On Mia, she discussed whether Mia falls under the women in refrigerators writing trope, where women are killed, abused, or otherwise harmed as a motivation for the player or protagonist. She argued that while she fit this trope, she doesn't "stay in the fridge," stating that she remains "strong, powerful, and utterly unfazed by her own murder," as well as a superior lawyer to Phoenix.

Miles Edgeworth (御剣 怜侍, Mitsurugi Reiji) is Phoenix Wright's long-time friend and first rival in the court room. He is temporarily playable in Phoenix Wright: Ace Attorney – Trials and Tribulations and the main player character in Ace Attorney Investigations: Miles Edgeworth and Ace Attorney Investigations 2: Prosecutor's Gambit. As a child, Edgeworth aspired to become a defense attorney, following in his father Gregory Edgeworth's footsteps. When they were children, Edgeworth successfully defended Phoenix in a classroom trial, where Phoenix was accused of stealing lunch money from Edgeworth himself. However, upon witnessing his father's death and watching in horror as the suspected murderer was let free, he gained a hatred for criminals and began studying to be a prosecutor, learning tactics from his mentor Manfred von Karma to always get a "guilty" verdict. This later earned him the name "demon prosecutor". Miles had not lost a case he was involved with until his first trial against Phoenix, after which he felt a need to defeat Phoenix. By the end of the first game, Miles would undergo some self-reflection, and eventually cast aside his traumas to become a more well-rounded and just prosecutor, instead of just chasing guilty verdicts.

Edgeworth's spin-off game Ace Attorney Investigations was originally going to star Ema Skye, another character from the series, but due to fan response, they went with Miles Edgeworth, a more popular character.