@sebaasdf They're ALL at fault, and all equally so. After their mother's death Miya shut herself off emotionally from her father Kosuke, and pretty much was unable to come to terms with her grief, and quite possibly blames Kosuke for the Mother's sudden passing. Upon being coerced into going back to school by Koto, she then runs off for a day with some boy who basically didn't know the entire story and thought he was protecting her from her burdens while subsequently turning off her phone, which only caused more strain upon the Father, for obvious reasons. She does admit to "trying" to understand / reconnect with Kosuke, but in honesty she was pretty passive about it, and waited for Kosuke to do the heavy lifting / repair their relationship when she was the one who cut him off. Unsurprisingly Miya and Kosuke both have the same trauma and subsequently the same needs, AKA, someone to lean on / understand / carry their burden, and while Miya has that boy (or at least for as much as I remember him) Kosuke has no one else except Koto. Understandably Miya is extremely confused (because to her it seems like Kosuke is a pedophile trying to run off with a minor) and again, unable to come to terms with the situation.
Kosuke tried his best to play the responsible adult in the Kosuke / Koto "relationship" constantly refusing Koto's advances. Ultimately however, stress from his day to day life, from his unresponsive daughter, increased workload and subsequent burden, needing to keep up appearances as a working adult, the loss of his wife, and having way too much to do in way too little time, Kosuke ultimately cracks under pressure and gives in to Koto's advances. Everyone has base needs that must be met, and Kosuke is no different. It's only been about a year since his wife passed, and considering how he's sort of resentful of people who've got loads of free time in Chapter 1, it's a safe assumption that he hasn't had time to process / come to terms with his own grief. Like with Miya, Kosuke needs help and or someone who understands what he's going through, and Koto provides that. Sort of. He is still bound by societal convention however, and like the officer says in the previous chapter, even if their actions are consensual, society will not see it that way, and with good reason. The previous chapter has him realizing that he loves Koto, and is now willing to go to great lengths to keep her by her side. For an entire year he's been alone with no one to help him or listen to his burden, so it's not surprising that he's willing to risk a lot for someone that was willing to stay by him.
Koto IMO has the strangest motivations of all of them. Truthfully I'm unsure of what precisely her motive is, whether it be because she sympathizes with Kosuke, genuinely likes him, or believes he can give her the freedom she's been deprived of all her life, Koto's motivations towards Kosuke are a bit fuzzy. Koto isn't without blame however, as her upbringing has caused her to seek something or someone who can not only sympathize with her, but give her the freedom she desires. The aquarium chapter where she highlights the differences between the mother and father was her true awakening to craving freedom from her mother, and Koto seems to have no one except Miya and Kosuke. While she does sympathize with Kosuke, Koto seems very, very willing to chase after that freedom, at the expense of Kosuke's reputation and quite possibly his financial stability. The previous chapter highlights that she's been trying to break away from the mold that her mother has shaped her in (thus dropping the "good girl" act) and being rejected seems to have flung her to desperate measures, and her running away now is her ultimatum for Kosuke.
All three of them have genuine shortcomings and flaws that they're desperately trying to overcome, but for the moment we'll just have to see how this plays out. It's very, very twisted how the 3's relationships are intersecting. Had the three of them sat down to talk for a while, and for each of them to genuinely listen, things might've turned better sooner.