I forgot, what did the father do again? Whyd he leave?

It was never properly explained, but it was likely that he got into huge debt or did something that would "dishonour the family", which is why he said he was too shameful to come back.

there was no explanation, just vague implication, but from what i gathered he was a good and loving but cowardly and irresponsible father ? basically showing how relationships are complicated because often its not black and white. keeping in line with the title, its a coming of age story, so i suppose its focusing on lcy's reconciliation with the father in his memories who impacted him as a person and the real person that he is/was. and that he can still love him while being deeply affected by his absence, and that he can forgive him for himself, and feel all those emotions.