I want to know a good word for a buddy who only helps, but never takes any help, and he is with you only to help you.
You may use altruist.
unselfish regard for or devotion to the welfare of others.
Cambridge Online Dictionary defines altruism as follows:
willingness to do things which bring advantages to other people, even if it results in disadvantage for yourself.
Wikipedia has a page for altruism as well.
Perhaps you could call that person a "guardian angel", meaning:
a person who looks after or is concerned with the welfare of another.