Leo's epiphany came in the form of realizing that Hazel's story was basically Persephone's story in reverse. While Persephone was pulled from the springfield into the underworld by Hades, Nico pulled Hazel from the underworld into the world of the living.
And that gives him hope; that maybe, just maybe, she'll be with him for a long time.
Leo avoids the word forever because, well, they're demigods. And even if they survive this, there will always be something around the corner, some dumb, chilling prophecy awaiting them (and only them, why can't it be someone else for a change?) some apocalypse in the works. And forever is pretty much a foreign concept to them.
So as much as Leo silently hopes for forever, he'll settle for a long time. But the only thing that seems everlasting is the pain.
Leo can never entirely believe that Hazel is a daughter of Hades, or that for over ninety years, she was dead. He just can't. Not when she is highlighted and outlined by the sun the way she is, almost as if the heated rays are embracing her form and passing through her. Not when her eyes glitter and shine so vibrantly, not once, nor ever, will Leo be able to connect Hazel to death.
But there are times when Hazel is with Nico, and he can kind of see it, if he squints. Because there is really nothing identical between Nico and Hazel, gosh, If Leo didn't know them he wouldn't even assume they lived next door to each other.
However at times Leo can see Hazel's golden eyes grow a shade darker, and her mood be affected by Nico's. But then she comes back to him, and she's herself. Sunshine and smelling like cinnamon rolls and Leo's own grease he uses and she's his again.
Someone like Hazel doesn't belong to anyone. Leo knows that. Still. . .
Leo's mind wandered a lot these days, to keep his thoughts away from the painful truth that a certain son of Poseidon and daughter of Athena were no longer among them. He wonders, with a cringe, what it must've been like for Hazel, swimming amongst thousands of other dead souls in the Fields of Asphodel. And the vision he gets is blurry; unseen. He can't imagine that. He just can't.
He doesn't ask Hazel what is was like, either. He would never. Still, he wonders if she's thought about it. And if she ever worries that she'll go back.
Hazel is more like Persephone than Hades, or at least in Leo's opinion. How the sun seems to stalk her, and how her hair seems to dance and sway around her face like some flower Leo's never seen but knows must exist somewhere. She's more like the goddess of springtime than the god of the underworld.
He feels that nagging desire to possess her, to keep her by his side, to let the heat escape him and press itself onto her, to see her skin glitter and shine like the diamonds she summons.
Or maybe, he's more like Hades than Hephaestus.