This is supposed to follow along with the drama cd canon, as far as I know about it (which is still too little ;_;). There's scene where Rei and Shin talk about their regrets which is translated at the aarinfantasy thread for Saint Beast (#737, sorry I can't actually link to it). I found it sweet and sort of decided to follow it up in a fic, set sometime after they become freed.
After so long sealed in darkness it was almost beyond comprehension to be once more in the world, reunited not only with Shin, Goh, and Gai but also the two brightest angels of Heaven from whom because of their own betrayal they were sundered. Once more the six Saint Beasts were united in strength, but they were also lost in their purpose, at a crossroads now, unsure how to react to the current reign of Heaven but still carrying wounds from past that needed to be answered to. Though they had found a place to lay low for a time while they decided their course, Rei couldn't help but be filled with anxiety over the both thrilling and nerve-wracking feelings he had to face during this period of quiet, impatient waiting.
Being with Luca again he was overwhelmed with emotions he no longer knew how to express. It was confounding how things could be so much the same and yet so different. His first instinct on being reunited was to apologize for all he said and did while bewitched into fighting against his friend, but Luca had dismissed it so easily, as had Judas, saying it was never a matter for forgiveness. Neither thought it was right that resentment should come between the Saint Beasts. As it was, although his guilt was not abated—could not be so lightly—Rei had at least hoped he could talk to Luca with freedom now. However, though there was no lack of ease in their interaction it seemed so challenging to say the things he most wanted to. To ask questions that might be hard for Luca to answer, about everything that had happened over the thousands of nights and days since they were both sealed away. Moreover, in spite of the increasing ease the Saint Beasts found in working together again, Gai with his usual tactlessness and high spirits overturning any lingering formalities, Goh and Judas carefully working out what the correct action is to take next, himself chiding Gai for his boorishness as if no time had passed since their imprisonment—no, no matter how simply they could fall back into place despite the memories that haunted them all, it wasn't the same thing speaking with Luca amidst the others and being with him alone.
So it led to this. While the others steadily one by one began to retire for the evening Rei remained where he was, silently hoping Luca, who was resting against the window ledge looking out against the night, might linger awhile. Rei couldn't help but fidget a little in impatience and anxiety, but with the energetic Gai finally giving in to tiredness, the chance Rei had been waiting for was realized. He abruptly stood up started to walk towards Luca, but stopped himself short, trying to think of what to say first. He held his tongue for a moment, unsure where to begin, before settling on another of his old regrets, one that might not be so difficult to tell.
"Luca," he said, drawing his attention from the world outside.
"Yes, Rei?" Luca responded, his voice quiet and gentle as it always had been.
Rei hesitated to start, but forced himself to be forthright as was his usual manner. "Do you remember our first meeting?"
"The Day of Conversion."
"No," Rei said, blushing a little before deciding to go on, "before that."
"…yes," Luca said, just a hint of a smile on his face.
"What?" Rei cried out, turning his back to Luca to hide how flushed he was. It was not the answer he wanted. He had wondered about it for some time thanks to Shin but now that it had come out it was really embarrassing to think Luca had always remembered that image of him from when they first met even though he was too gracious to say anything. "So you knew all along?"
"It's not so easy to forget, Rei," Luca replied, that subtle restrained amusement still lingering in his tone. He came to Rei's side and gently took his hand, trying to encourage Rei to meet his eyes. "How could I forget the little angel with violet hair who was crying in the woods all alone and lost?"
Rei's face was positively crimson by now. He stared at the ground fixatedly and mumbled, "I'd hoped you'd forgotten."
"Then why did you ask?"
"Because I…I never thanked you for it, and that felt dishonest. I went looking for you afterwards truthfully, but when I found you I was too embarrassed to approach you, thinking how I must have appeared. So I kept waiting, until the day I got my wings and then thought I could finally speak to you without appearing so foolish…but then you knew all along! To think how silly I…" Rei trailed off, almost regretting bringing it up at all, in spite of his former resolve.
Luca's hand moved to Rei's chin, guiding Rei to look at him once again. "I'm sorry then, that you felt like that," he spoke, his tone wistful. "To think how much more time we could have had."
"Luca," Rei sighed, feeling overwhelmed by emotions that had been suppressed for so long. Such close proximity between them was maddening. In a moment of impetuousness he suddenly leaned forward and caught Luca's lips with his own.
Luca stumbled back a step, caught off-guard by the sudden weight against him, the heat of Rei's body, the passion in the sudden kiss, but quickly everything else dissipated, the burden of history; the confusion of feeling; the constant thoughts that he weighed out so carefully. None of it applied. The feeling of his fingers tangling in Rei's hair, the pressure of Rei's lips against his, the sweetness of the moment, and the longing that it might last eternally, a moment outside of time, this was all he could think of. How unparallelingly wonderful his lover's embrace could feel. No amount of separation, no arrogant gods, and no curse could take away the fact that in this moment, their love was as it was always been meant to be.
Gently Rei pulled away, ending the kiss what felt far too soon. As they stood there, staring intently with no sound but their breathing to be heard, Luca lightly brushed his fingers against Rei's cheek and read what he was thinking. This time it was not because he needed to, rather, after so long a time he simply wanted to feel Rei's unadulterated thoughts again. Rei gasped at the sensation as he always had before, but this time instead of veiling his gaze he looked expectantly at Luca, waiting for him to speak.
"Oh, Rei," Luca said tenderly, caressing his cheek. "Me too."
He leaned in and embraced Rei once more.