We don't know why she left...? And leaving a child behind is not always cold-hearted. Sometimes it's done FOR the child. Also, women can have ambivalent feelings about their children just like men may do. It's not their fault any more than the men's that they can't connect to their children. Alrighty?
My point was that they portrayed her as a warm loving mother and then the next thing she's walking out on him, and that it's a common theme in manga, being abandoned by parents in such cold-hearted ways. Nothing much deeper than that. I've seen a lot of parents in my 48 years, from down-right shitty to over-the-top spoil mongers, so I'm quite aware of the myriad relationship possibilities between parent and child. Alrighty?

Chap 6, his mom made him things "with love" but she walks out on him as a young boy? Not love at all. I noticed this is a common theme in manga too, women cold hearted enough to leave their kids behind.