Typical mangadex incel reactions- the guy never does anything wrong! Women are always to blame! No one is blaming Tatsu in this just pointing out his part in the relationship having issues.
Honestly, it's never one person or the others fault fully. Relationship usually die because problems both people have. Tatsu is to partly to blame because he has a lot of trouble seeing past Kotoka's hair and understanding that Kotoka wants him to love her for more than that and more importanty, in spite of that. She want him to love her for who she is, not because of his hair fetish. That's why his mother said she doesn't want him to make the same mistakes she did- she and Tatsu's dad divorced because she never let him feel like she loved him for anything other than his hands.
Kotoka is partly to blame for not being fully clear on what she wants, her own inability hold back on her fetish, and making ultimatums instead of letting things go more naturally. Sadly, we didn't get to see more of her point of view on it so we can't dive as deeply into her issues. She seems to be putting distance between them to try the old trope "absence makes the heart grow finder", which is a huge gamble, but her up and leaving without an explanation was wrong.
But that's how relationships usually go when both sides have different views of the future and things don't click, people make mistakes and do stupid things. You either overcome them or break apart.