@newershadow at first your theory seem a bit off based cause i assume right now minamida's age would be around 22-26 so 13 year ago he would has 9-13 yo which seem impossible atleast he must be still in the hospital cause his brother coma for 10 year and never mention that he's not in the hospital anymore
but i went back reread it and yeah politician wife died 13 year ago and shizuku's mother accident happen 15 year ago so it might connect in one way or another
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and another theory is older minamida become evil spirit and trying to set crime according to story in his novel he wrote(perfect crime), because his novel is not complete yet and want his lil bro to read it(but it's too roundabout he can possess someone write and send it to his lil bro his lil bro is kinda smart too he might figure it out)
but this theory wouldn't be true if we don't know the content inside his novel
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some theory in internet said the culprit is his mother
mother last wish might be able to see again or turn her body back before incident which make he blind and disabled
she angry that hibiki face resemble the dad's cousin that in the car on the incident day(she must know that cousin plan to kill her but never mention before in any chapter)
and his father wish is to erase shizuku memory about his mother kill hibiki (why did he also get kill maybe he try to holding mother back) (it also answer the point of hibiki got her leg cut because mother can't use her leg either. but why are you so angry mother-chan lmao)
(actually this plot seem bullshit unacceptable for me because then who take shizuka pic and only hibiki can heal the other because each shinigami have unique ability(but in some chapter mention that saaya learning until she got erase memory ability maybe ability can be learn and not actually unique))
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actually author still hold a lot of plot to play like
what secret sayaa holding?
what happen next after shiroha become shinigami?
why only shizuku alive?(in this case both dad and mom must not be culprit so one of them can use last wish to save shizuku like wish for culprit to not kill shizuku and another wish to erase his memory during that time/or erase everyone memory that witness the incident so culprit will confuse try to get evidence and send to police then run away)
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Summerize : no more open ending/axed pls there's no theory that give me 100% sure that who was the culprit