Who Qualifies To Be A Best Man Or A Best Lady?

Just curious!!

Who fits to be either a best man or best lady?.

Does it have to be your family member, or a friend or someone that has done great thing for one in the past or one just pick it?

Just want to know the right parameter to consider before picking

Please contribute!

I has to be someone you cherish so much.

Someone you can personally single out (One in a million)

Someone who can stand for you anytime anyplace anywhere.

Someone who knows the meaning of true friendship.

He may be you family member, he may not be your family member it doesn't matter.

It could be any, depends on how dear they are genuely,

Such person may not really know but you have ur ways of counting what make who the best