I’m guessing it’s a pet. That draws a strong parallel to what was happening in her book, and when Asa asked if she’d lost someone important the word she used in Japanese was one that would only be used to describe people.

I’d tend to guess too that it’s something conceptual. At this point, I expect that I won’t figure it out and will be surprised when (if) it’s revealed, but then it will fall neatly into place and make perfect sense.

My first thought was that it was her sister that she lost. As in their “sisterhood”, not as in her “death”. But earlier when they had their falling out that made her hate her. Maybe they were close once. I’m not sure if it’s a realistic depiction of a sibling bond, but it’s something that would fit into the META of the show. It’s what I would do if I were a writer when trying to spin the story around a central plot point.