a friend in need, who is always willing to do something useful for you, who can give good advice, who can keep secrets

a friend who is extremely virtuous and for whose sake you dare not do something bad, for fear of displeasing or upsetting him or her

a good enemy and I hope to make you mine

an extension of you, only it's the part of you who gives a shit all the time

a person who participates in all your endeavours and stands by you when you are in a crisis

a person you call to bail you out when you are in jail