the endless turnabout
The below is simply an overview; for the details, I recommend playing the games, which are much more rewarding and entertaining.
Mia Fey's first court case deals with the murder of Valerie Hawthorne, and through it goes up against Dahlia Hawthorne for the first time. Though Mia shows through brilliant deductions that her client, Terry Fawles, could not have committed the murder, the case ends in tragedy due to Dahlia's manipulations. Mia is left with a huge wound in her heart that leaves her unable to take on any cases.
But Diego is far from satisfied with the verdict. He and Mia take it upon themselves to find out what really happened, and their efforts are realized when Dahlia agrees to meet with Diego. But in doing that, he seals his fate. Dahlia poisons him and hides the evidence with one unsuspecting Phoenix Wright. Diego goes into a coma, one that he would not come out of for a long time...
Eight months later, Mia takes on her second case, meeting Dahlia once again in the courtroom. This time, she succeeds, and puts her behind bars for good.
For three years Mia continues to work as a lawyer and opens her own law practice, taking on Phoenix as an apprentice of sorts. But she is murdered shortly afterwards, leaving Phoenix to muddle through by himself. Since she is of the Fey line who practice spirit channeling, Maya and Pearl help her return to aid her protégé when he's in a pinch, but for the most part she is in the great beyond.
Two years later, the scent of coffee awakens Diego from his coma to a world without Mia. Diego loses sight of who he is and takes on the name Godot, becoming a prosecutor for two reasons: to do battle in the courtroom with the man he believes should have prevented Mia's death, and to protect the one who was most precious to Mia: her sister Maya. In the first, he is unsuccessful, since Phoenix always manages to turn things in his favor, but Diego eventually realizes that wanting that is merely a reflection of his own shortcomings. As for the second, he successfully defends Maya against Dahlia, though at great personal cost, seeing as he committed murder to do it.