(As given by Rupa Gosvami in Bhaktirasamrta-sindhu)
The definition of a pure devotee can be summarized thus: his service is favorable and is always in relation to Krsna. The symbol of devotional service in the highest degree is Radharani. Krsna is called Madanamohana, which means that He is so attractive that He can defeat the attraction of thousands of Cupids. But Radharani is still more attractive, for She can even attract Krsna. Therefore devotees call Her Madana-mohana-mohini--the attractor of the attractor of Cupid. To perform devotional service means to follow in the footsteps of Radharani, and devotees in Vrndavana put themselves under the care of Radharani in order to achieve perfection in their devotional service.