Friend is the most intimate and important relation. This is differentiated by the fact that this is not blood relation. Thus friends are by your choice unlike blood relation like parents, siblings , cousins, uncles, aunts that are not by your choice but by accident of birth.
Friend is one you may communicate and otherwise interact freely without inhibitions. If there are any inhibitions, there is something lackin in friendship.
It is often said a friend in need is friend indeed. I do not agree with this definition. A friend may not be competent enough to help you in a situation. even then if he is sincere to you, he is your friend.
Absence of inhibitions should enable you to demand whatever you think, from your friend. The friend may unhesitatingly accept your demand or even unhesitatingly refuse. True friendship is not any way affected by making demand or refusing it. This is simply absence of inhibitions and hesitation in mutual interactions.
Friendship need not be just physical intimacy. It is more a matter of mental identity. Lord Krishna had a friend sudama, whom he loved and had intimate friendship despite vast difference in status.
Lord Krishna had relations with three women. Rukmani-wife, Radha- beloved and Drupadi- friend. He had no romantic relation with Rumani unlike Radha and Rukmani. Drupadi was his friend. Nobody except Lord Krishna was of help to Drupadi in her hour of crisis. This is because of friendship. There is no relation like friendship.