Takou no Koori Hime wo Tasuketara, Otomodachi kara Hajimeru Koto ni Narimashita - Vol. 1 Ch. 3.2 - Her usual self

[Kaarme]

Well the father wouldn't be wrong if this was a advise to a boy. For girls though the opposite usually works better. As for how she interpreted the "never show weakness" advise that's not quite how it works

The father wasn't wrong, but the problem is a little kid had no idea what the advice means. If the dad isn't around anymore, she can't ask for clarification, either. Instead it has become a mantra in her mind. She never questioned the meaning, despite growing up.

For example being cold to everyone and avoiding having any friends if not strength, it in itself is a weakness. So, by being isolated and unable to communicate, she's precisely having a weakness on full display. As a simple example, a child with a bunch of friend won't easily become the target of bullies, while a lonely kid will.

In her specific case, she's bad at English, but in her religious belief in that mantra, she can't seek help anywhere (showing her weakness to others), so she will remain bad at English. Is that supposed to be some kind of strength? That's absolutely not what her old man must have meant, unless he was a chuuni idiot.

Alas, the MC dude has the personality of a tuber of potato, so he couldn't give her this sort of advice.