@KazumiSangaku
Don't get me wrong. I didn't say she intended to betray him, and I understand that she is not really the same as active Koto she is when she isn't with her abusive mother. The author drew her blank eyes of a puppet to clarify that. But nevertheless from outside pov, including his pov, she didn't defend him for what that was partly consequence of her actions. That is equivalent of betrayal, even it's unintended. After all, when she has to choose her side, she went with her mother even after reaping benefit from him. The author even drew symbolism of that.
And unfortunately, as you said she is traumatised by her mother, but everyone except him, especially that policewoman blame all mental trauma to him.
Her submission to her trauma can be interpreted in two different ways. First is that she is a teenager who still not equipped with mental immunity to overcome that trauma, which would excuse her from her actions, but in the same reasoning would put other two traumatised adults that can't overcome their at fault. Another way is to see her as a people with developed mentality equal to an adult, but admitted that there are some trauma that even adults cannot overcome easily, which would excuses all of them. I'm pretty certain that most adults here knows which ones fit the reality, but also knows which way society and laws works.
Edition
@starlightcitypro
I'm tagging you there to say that while you are right about what they did and what they can do, you are just putting Kousuke in the same bin that psychopathic pedophile belongs, but I disagree that it is right to easily just put him there. That would be injustice of listening to just one side.
Let me clarify, he did wrong, no argument here. As you said, everything would not happen if he stopped her. The problem is, is he mentally capable of stopping her in his situation?
Everyone in this manga is heavily traumatised, both daughters and their parents. In a way, everything would not be this bad if just ONE of them capable of doings the right thing despite of his/her trauma. Am I wrong here?
If Kousuke is 100% wrong and is a true pedophile here, he'd also readily flirting with other girls. But thing went the way it was because Koto being uncommonly bold first and continually insistent. I'm not victim blaming here. He tried to stop her already but she isn't normal girl that would be stopped with just that, but she was seeking someone to vent off her trauma. That's pretty much justifiable, especially considering she is still young. Unfortunately, her chosen person is also traumatised and cannot properly guide her, and can only be source of her validation that escalate badly. In a way, both of them exploited each other weakness, unintended as it is.
What Kousuke did that is most wrong in my opinion is that he didn't seek a psychotherapist. But considering his character of trying to be normal in eyes of other, which only intensified by his stress after his wife passed away, to the point that he didn't even thinking about asking other advice for his daughter problem, I can't really blame him there. It seems that being narrow-minded and unaware of his problems are parts of his problems.
