#tbhk chapter 131 spoilers !
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If tsukasa doesn't sacrifice himself, then serial killer amane will happen.
If tsukasa sacrifices himself, the original timeline happens.
Nene is a kannagi. So in the end, she might sacrifice herself to prevent any of the twins' fates.
It's not talked about as much but I think, as a society, we should encourage more people to actually learn sign language. Not only is it beneficial to an individual person, but it is also beneficial when it comes to the mute and deaf community ^^!