the turnabout, once and again
Phoenix Wright, the ace attorney who takes on cases at extreme disadvantages and turns them on their heads, strongly ties Mia and Diego together. It's through Phoenix's trial that Mia is able to take down Dahlia Hawthorne thanks to Diego's poisoning at her hands. And it's Phoenix that Diego faces in court under Godot's mask. But what is it like from Phoenix's perspective?
Mia
It takes Phoenix a while to trust Mia, even though she's his lawyer and his only ally in his trial, due to how tightly Dahlia has him wrapped around her finger. Eventually, though, he sees the truth and is set free from his chains from Dahlia and can get back on with his life. Which, in Phoenix's case, means becoming a lawyer so he too can help people the way Mia does. Once he becomes a lawyer, his relationship with Mia becomes one of mentor and student. Her death does not break this bond but only strengthens it in that she can continue to be his mentor via Maya and Pearl. Through her guidance, Phoenix eventually becomes a skilled defense lawyer.
But what's more important is how Phoenix teaches Mia. It's through Phoenix that Mia first feels the joy of success in defending a client rather than the agony of utter failure she felt with Terry Fawles. It's Phoenix that does what Mia could not accomplish in life, such as putting Redd White behind bars. He carries on her legacy of helping those who need it to great success, and on a more personal level looks after Maya and Pearl. For all of this, Mia is grateful to Phoenix.
Diego
On the other hand, Diego feels the opposite. He blames Phoenix for Mia's death, unable to accept it. Of course, he doesn't tell Phoenix this. Instead, Phoenix is left to wonder why the guy in the mask hates him so much. It's extremely petty behavior for Diego, but as Godot, he can never tell Phoenix why. To him, that would be admitting defeat and accepting that there's no one he can rightly blame for Mia's death but himself. Instead, he avoids it by mocking Phoenix — he won't even say his name properly.
Godot never apologizes to Phoenix for how he treats him, instead only acknowledging it when he explains why and what he did. But the sentiment is meant in the cup of coffee they share at the end of the long final case. Phoenix laments to Mia that he couldn't save Diego, but Mia sees what Phoenix can't and knows he did.