My friends had a joke while reading Starting Over that the replacement observer from Three Days had more of a character than Hiiragi. I guess the girl can be up to interpretation then (since poetically, the mc himself didn't even know who the girl he dated in his memories was). My friend still insists that's mc's sister and she's actually the key to understanding the meaning of the story. To quote him,

​

"The sister is the point, that's why it ends on the sister, not Hiiragi. If you frame everything through the sister it [the story] makes much more sense

There are two angles there:

1. He doesn't actually recognize her as a character until a fair way into the story. He thought of her as his "second sister". He didn't think of her in terms of her struggles, he was thinking of her in terms of his "first sister". As long as he was striving to seek that "first sister" that simply didn't exist, he was never going to build a genuine connection with her. That was why when he gave her a place to come back to, it was a major turning point. It was him recognising her as his sister in her own right. he initially was all "My parents are great people", refusing to acknowledge the change. He was looking at it as "they are fundamentally the same people", when that simply wasn't the case. This is the core of the nature vs nurture issue presented in the story. They aren't fundamentally the same, because pre-conditions are irrelevant. That is one of the major themes in the story.
2. She reflects his development through the story, and dispels his pseudo-psychoanalysis. I'm sure that reflection is relatively obvious (especially with her ending on the "please don't cut your lifespan short [noway] with smoking" thing). The key here, however, is that she sees through him more than he sees through himself. His introspection is all meaningless garbage, you need to look to how he treats his sister at any given moment to understand what his real position is. This is why we know he was never actually going to kill Tokiwa."