bittersweet love
I was moved by her...
At some point after the events of the kidnapping case six years before the start of Trials & Tribulations, Diego and Mia start dating. I doubt it's before that case; they hardly talk to each other during it like they're together, Diego's teasing aside. The case broke Mia's heart and wounded her soul, and it was Diego who was there for her and helped her keep going. They were dating when Diego was poisoned and sent into a coma that he would not wake from in Mia's lifetime.
It's a tragic love, but it's one that affected them both deeply. I don't think Mia found someone else while he was comatose; at least, she certainly wasn't with anyone at the time of her death, or else there would have been some mention of a significant other. Instead, the only people shown affected by her death are Maya and Phoenix. Mia might have thought that Diego would have wanted her to move on since he wasn't there. But if there was one thing she knew, it was that even in his coma, Diego was still alive, because if she had tried to channel him she would have failed.
No matter what Mia does, though, he never returns to her, and instead opens his eyes to the world only after she's left it. He is lost without her — for all that he did, why is he punished by the loss of the one he loved most, the one who touched his heart? It was Diego who loved Mia, but without her, he isn't the same man at all. Instead, he becomes Godot. The one thing he can't do is let go of Mia's memory. Everything he does as Godot is because of her, from protecting her sister to battling Phoenix in court. His devotion to her never wavers, even when she's left this world and gone to one he can't reach.
If there is one person for everyone, then I'd say that Mia and Diego found it in each other. A woman like Dahlia would never suit Diego; it's a strong, sure woman like Mia that's more his type, even when she despairs. And Mia wouldn't want someone like Phoenix who would dote on her; it's a man who can let her be herself but still be there for her that she needs. Their shared occupation certainly helps, as they would certainly understand the pressures of the job the other faces and would be there to lend a hand. In fact, the only thing that would make Mia more of a perfect fit for Diego would be if she had the same love of coffee, but then again his obsession is unnatural enough.
There is no happy ending to this love story, but by the end of the game, Diego has found himself again. Like Mia, I doubt he could possibly move on (especially not with his inevitable jail time), but he knows that one day, he'll be with her again in death.