For the sake of clarity, it is usually better to give a fuller answer to this type of question. For example: "What's the problem, are you saying that you cannot remember your name?" "No, I cannot remember my name," or "Yes, that's true, I cannot remember my name." If you just answer "no," then the meaning could be unclear i.e. "No, that's not what I meant at all, I meant.............."

In answer to your excellent question, we just have to accept that it’s idiomatic, i.e., that’s just the way we do things in English. I believe it’s the other way around in some languages (such as Chinese and Russian), but in English, we do it this way. I suppose that we feel that whether we ask a question in the positive or negative, the meaning is essentially the same. In a real conversation, nonverbal factors would play a role. For added clarity, we can use the auxiliary verb in our answer: “No, I can’t,” or “yes, she is.” We could even omit the yes / no and just say “I can’t” or “she is.” That would actually be more like Chinese, wouldn’t it? By the way, your second question should be “He isn’t a girl, is he?”

I can understand why you are confused, but try looking at it this way: Unlike in some languages, the 'Yes' / 'No' response does not mean 'You are correct/incorrect' or 'That is true/untrue'. In English, the 'Yes' or 'No' response has to mirror the GRAMMAR of the sentence it refers to. In other words, if the response is grammatically negative, you say 'No', and if the response is grammatically positive, you say 'Yes. For example: 'You can't remember your name?' 'No [I can't]' 'You can't remember your name?' 'Yes [I can]' That isn't a girl, is it? ( A corrected version of your example) No, [ it isn't] That isn't a girl, is it? Yes, [ it is] Here's a little exercise for you. Put either 'Yes' or 'No' in the gap: You've never been here before, have you? ............, I haven't. It's my first time here. You've never been here before, have you? ............ I have, in fact. I came here once when I was a child. So he didn't pass his exam? .......... he did. The teacher re-calculated his marks and he got a pass. So he didn't pass his exam? ............, he didn't. He'll have to take it again. And, by the way, there's no such word as 'insteadly'. It should be 'instead of'.