I think you've confused yourself. "I met a kind girl yesterday." No confusion here.

1) "I met a girl who was kind yesterday" is correct. "I met a girl who is kind yesterday" is incorrect. How kind the girl you met yesterday is or will be is irrelevant to your statement that what happened to you yesterday, that is, you met a kind girl yesterday. I had the same quesiton (about "tense") when I learnt German and tried to compare them (German, English and Mandarin) on this issue. Luckily, later on I got a satisfying answer by a great italki member, fdmaxey. 2) Then to your "truth" question. You are absolutely right but be cautious, there lies a little translation between English and Mandarin: In English, the word "truth" can be either of natural law/objective existence (真理), a fact (事实) or an event (事件). That said, "Boys, let's take a look at the tellurion," said the teacher Confucius, "we know that our Earth SPINS counterclockwise and ROTATES around the sun." "You are wrong, teacher!" Tom Sawyer stood up, boldly, "The Earth SPUN clockwise when I hovered over Antarctica with my astronaut buddy Hadfield last summer."

"What I have learnt in English class is that truth always use present tense". This has to be one of the craziest ways of trying to teach English I have ever heard. It is the truth that I ate an apple yesterday, that doesn't mean I can say 'I eat an apple yesterday'!

I agree with Peachey. You could also say : I met a girl yesterday. She was kind to me. -- In that case, you are talking of her behavior yesterday without commenting on her ongoing personality. No confusion there either.