I think maybe, Kai-san was either another spirit(but a child) that Asuka met when he was a little, or was a childhood friend of Asuka that died. All solely based on what I think is going on tho, cuz I'm pretty sure when Asuka was "going through" Kai-san he was seeing a fragment of Kai-san's memory, so the child that looks like Asuka IS Asuka, we're just seeing it from Kai-san's perspective
I think maybe, Kai-san was either another spirit(but a child) that Asuka met when he was a little, or was a childhood friend of Asuka that died. All solely based on what I think is going on tho, cuz I'm pretty sure when Asuka was "going through" Kai-san he was seeing a fragment of Kai-san's memory, so the child that looks like Asuka IS Asuka, we're just seeing it from Kai-san's perspective
That's all my theory, waiting for more!!! ლ(´ڡ`ლ)