That's not to say that there aren't some interesting details spilled in the backstory presented here. The discoveries in Hishigata's research are both fascinating and deep in implication, that the strength of a psychic's abilities could at least in part be connected to body size, although it only seems to make a difference if the body is much bigger than normal human size. That the increased body size also tends to cause subjects to lose their minds is a big limiting factor, which explains why the Sisters would be so valuable to Hishigata. This also suggests why animated corpses were used in earlier encounters with the mecha: they could either serve as hosts for the special spirits Esther has mentioned or else might not be as susceptible to losing their minds. That this all stemmed from a side effort to develop a Level 6 isn't at all surprising, given the level-bumping nature of what has been encountered so far, though Hishigata's claims that he's only a couple of years from the point where “anyone can become a Level 6” is a pretty heady claim, since it would throw the whole power balance of the setting off.
What the backstory doesn't do is provide a strong foundation for a deep friendship between Esther and Hirumi. Esther basically got dragged into the association and was always reserved about Hirumi's enthusiasm, and the tragic nature of Hirumi's situation does not feel like it should have intensified that bond much. It also does not explain why Hishigata was exclusively experimenting with female
victims subjects when there should be plenty of male subjects of comparable age out there as well. Based on the way this series has gone so far, my suspicion is that there's no better reason than to provide more opportunities to show girls in lingerie, but hey, Hishigata was at least in part doing this whole project to benefit his sister Hirumi (unless I'm misremembering, this is the first time that relationship has been clarified), so maybe the subjects were chosen with preparing something for her in mind, right? Right?
Yeah, I'm not buying that one either.
At the very least, the backstory does at least partly explain where this whole story is coming from, though it feels like another few minutes would be needed to fully complete it. It also gives Esther a basis for finding some resolve to fight Hirumi, though what she's going to do with that mystic dagger is not clear yet. The cameo by the trio who terrorized the hospital in episode 1 was a nice little touch as well. Still, I think the series will be better off when Accelerator returns to the heart of the action.
A Certain Scientific Accelerator is currently streaming on Crunchyroll.