Wow, that is a hard one.
One can possibly start by saying that a good person is a person that does only do good things. Then, one would have to define what good things are. We have laws, but laws define mostly the bad things. Everything else is probably subject to personal opinion.
One could say that a good person is an altruistic person. But then one would have to prove that true altruism exists. And can a person, that is sometimes egoistic, not be a good person?
So, maybe the definition of "good person" should be based more on the opinion of people who are in contact with that person. I.e. if I often meet with CK9 and I find that CK9 is always so nice to me, than CK9 is a good person. In that case my opinion would be based on the facts that CK9 shows respect for me, helps me when I need help and is just there as a friend.
A further aspect could be that a good person is always fair to others, i.e. if it is to judge about others it doesn't give one person a greater advantage than that person deserves. Of course that judgment has to be based on the opinions of the people being judged about and also has to take into account possible consequences on people who are not directly concerned.
Furthermore, if I find that CK9 is a good person, someone else may find that CK9 is not a good person.