fated to meet but once in the cycle
The story of Tanabata goes like this: A farmer finds a robe that belongs to the goddess Tanabata. She comes to him and asks if he has seen it, and the farmer lies and says he hasn't but will help her find it. They fall in love and have their happily ever after until one day, she sees a piece of cloth that was part of her robe on the roof of his hut. Tanabata leaves him but agrees to return and forgive him if he weaves a thousand pairs of straw shoes. He cannot do it in his lifetime and never sees her again. It is said that they meet once a year when the stars Altair and Vega intersect.
Adapted from Wikipedia
The details don't exactly match Diego and Mia's relationship, but the story of Tanabata illustrates an idea that's seen often in romantic stories: two people who fall in love, are separated, and cannot meet again. The farmer in the Tanabata story cannot accomplish the one task he's given to see the goddess again. In the same vein, nothing Mia nor Diego do can return the other. Diego does not wake up from his coma during Mia's life, only returning to the land of living after she's left it.
That alone would make it a typical story of star-crossed lovers. What gives their relationship a twist is that Mia can return from the beyond through the Kurain Channeling Technique, at the cost of her medium's body. That is how Mia and Diego meet again — across the courtroom, under different guises — Mia in Maya's body and Diego as Godot.
And in those forms, they cannot be together, even though they're in the same room, the same world. Godot surely knows it is not truly Mia, and he isn't the Diego Armando he once was. There's simply no way they could be together — Mia can only be there through a medium, after all. As in the Tanabata story, they can meet only under special circumstances, and then only briefly. It's a tragic love, but at least their hearts are cleared after the last case.