He didn't know why he watched her. But of course he did know, he just didn't want to admit it – there was loneliness in her eyes, thick like the cream on his cake. Pale, half-forgotten loneliness, the tiniest spark of blue. Of course he didn't really see just a piece of blue in her ocean-blue eyes, it was just a tiny piece that felt blue to him.
So yes, he did know why he watched her smile and why he stayed long past his welcome when she wanted to be with Light. He knew why he longed to touch his spidery fingers to her cheek and say 'it's okay' – but he didn't. She wouldn't have wanted it, for once.
He wouldn't have wanted it either, in retrospect. What he did know, though, was that he wanted to give her a piece of him, before he went. The nervousness in Lights behavior hadn't slipped him, his glances at his watch hadn't either. His time was running out. Long precious second were spent mourning for Watari, then thinking about a way to show her what was behind his own eyes, that no doubt carried a blue spark of their own.
Finally, he settled on the softest smile, an unpracticed smile he had never seen on his face before, he just had to hope that it was enough to get it across. Her blushing cheeks, her eyes, wide-open and oh-so-soft. She shouldn't be here, but he had called her anyway, knowing he would die, wanting to watch her, no matter how selfish the request. It didn't matter anyway – he wouldn't … couldn't bear watching her get locked up as the second Kira – dying and leaving his successors to the job was the only way he could protect himself from having the tiny blue spark in his own eyes take over his whole being.
He knew that one day, she would realize it – she wasn't stupid. She'd see, one day. When she did, she would follow him, he knew that as well. Just this once he wanted to be selfish. Just this once he wanted to hope that there was a life after death and that he would see her again and he would be able to tell her all the things he had concluded about her; tell her that her smile broke his heart.
Just this one time he didn't want to do the right thing, the just thing. He would trade all the lives he saved as a detective and wager them against the selfishness of letting her die, in the hopes that there was a merciful god, somewhere.
Because that tiny blue spark would consume her just as his did him, he knew all too well that she would follow. All he could do was smile and collapse and die, with the picture of her face as his last thought, knowing.
Knowing that one day she would hit the pavement with the same smile he wore the day he died.