Kairi had always loved mysteries. She'd loved reading them and trying her best to figure out the answer to the riddles before the detective heroes did (and had usually succeeded, too). And since Namine was a part of Kairi, she loved mysteries too. There hadn't really been very many mysteries for her to figure out in Castle Oblivion, however- she just did what she was told because she was too frightened not to. But now she worked with DiZ, doing her best to fix Sora's shattered memories, and that was almost like a mystery. Or maybe it was more like a puzzle, really, but she certainly preferred to think of it as a mystery, because those were more fun.
Sora's mind had been a blank slate to start with, after she erased herself from it, and she'd had to find the perfect memory to start with, to base the rest of her solution off of. Just like the detective had to find the first clue, the first bit of evidence to start making a case from. Namine had found it, of course, after rifling through all her drawings and finally deciding on the day Kairi arrived (just a little girl, lost and alone, with a bright and beautiful smile and hair that gleamed sunset). Most of Sora's memories were based around Kairi, after all. Not her. Never her.
And after that first memory (clue), Namine had searched all over for more related memories (bits of evidence) that she could use to link to that one and form chains of memories that made up Sora's life (her case). Memories of Kairi's smile, of their raft, of him and Riku playfighting while she cheered them on. Evidence and clues that hinted at what his life really truly was and should have been. She found memories of Kairi's lucky charm, the promise he had made her, everything that made Kairi Kairi and Sora Sora. Namine found those clues and pieced them together in order to form a working hypothesis about whodunit and why.
So, who had done it?
She had, of course.
But she was fixing it now, so didn't that make everything okay?