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Quote:

Originally Posted by Shin Henshin

A theory a friend and I have is that Yume's sister may have died in a prior kaiju attack during the events of SSSS.Gridman.

Mainly because we don't know how she died (afaik so far) and the subtle reuse of certain backdrops and scenes from Gridman most likely signifying that this is the computer world still, just after the events of the first series. Not to mention how Yume seems to act when encountering kaiju and looking over the wreckage their battles leave behind. It's not overt, but things like her prolonged silence at the end of their first battle and how she looks over the destroyed buildings and such in episode 2 makes you think...

Interesting, I was about to say "Wait, if Yume's sister died in SSSS.Gridman, wouldn't everyone's memories of her would've been erased by the Custodial Kaiju?", but then I remember they were all killed off near the finale.

All we know about her death so far is that when Yume was in elementary school, her sister accidentally died on the Flood Gate we see Yume standing on in ep 1. There's also this importance on September 21st, as the day was marked as "Recital" on the calendar in her sister's room, and we hear her invite Yume to come.

But none of this information actually contradicts your theory.