Well as you can see, I came away with a different take on "characters."Well, when "characters" is used in the plural in this conversation, I believe it's being confused with either "character traits" or "characteristics."
I think this native speaker used the word "character" in the sense E (Japanese teacher) was using out of kindness. Thoght it was a wrong choice of word, he chose not to hamper the flow of the confersation. Thank you, now I understand that E should have said "three characteristics are needed to be a good teacher" instead of "three characters." Now I know that one person can have only one character, but can have many characteristics.English."S" doesn't seem to have a clear grasp of the word "character."