How do I define " a good friend" ?

People can make many friends in anywhere. However, a good friend has jst only one for each person. Why do I say that because the definition of a good friend is that he or she could help you when you meet any problem. Also, when you need to complain somthing, you can talk with her or him. She or he might be your waste bin, and when you have something interesting, you will let her or him know at first second. For example, a few years ago, I fell down in the school's restroom, my teacher took me back my apartment. After that, my good friend Sandy had been helped me to make the three meals everyday for two weeks. Also, she slept in my apartment to take care of me at first week. I was really appreciation to her. She is my only good friend so far. I think that everyone has a good friend as me. Do you have one?