Most regret losing.
My memories.
The ones from about 1977 to 1983 or '84.
There's too much I can't really say on the subject, for too many reasons- half of them guesses. I know I was dead, and that I wasn't a Shinigami then, but that's pretty much it.
I'd like to be able to remember how I died, for one thing. And how I ended up here, even if I'm not quite so attached to what I spent the first few years here doing. For that matter, I wish I were sure when the first time I met Tatsumi or Tsuzuki or Konoe was, instead of vague impressions of things in the break room.
I don't imagine they were anything particularly... whatever-- but they were mine. And I'd rather everything in my head answered to me. At the very least, I wish I knew why I don't have them.