A pure devotee is he who loves Krishna, without any material desire. Such pure devotees are very rare. Even if they have got some ulterior desire, that will be removed very soon, because they have taken to the pure process of Krishna Consciousness.
In the preliminary phase of spiritual life there are different kinds of austerities, penances and similar processes for attaining self-realization. However, even if an executor of these processes is without any material desire, he still cannot achieve devotional service and aspiring by oneself alone to achieve devotional service is also not very hopeful, because Krishna does not agree to award devotional service to merely anyone. Krishna can easily offer a person material happiness or even liberation, but He does not agree very easily to award a person engagement in His devotional service. Devotional service can in fact be attained only through the mercy of a pure devotee.
Krishna loves to hear the glorification of his pure devotees.
A pure devotee is kept in the hands of Krishna just like a precious jewel. When you hold something precious in your hand, you are very careful, and similarly, Krishna holds the devotee and takes care of him.
Association with a pure devotee for even half a moment in this material world is the greatest treasure for human society.
One has to associate with a mahatma who has accepted Krishna as the supreme source of the entire creation. Without being a mahatma, one cannot understand Krishna's absolute position. A mahatma is rare and transcendental, and he is a pure devotee of Lord Krishna.