Radha (Devanagari: राधा, IAST: Rādhā), also called Radhika, Radharani and Radhikarani, is almost always depicted alongside Krishna and features prominently within the theology of today's Vallabha and Gaudiya Vaishnava sects, which regards Radha as the original Goddess or Shakti. Radha is also the principal god of worship in the Nimbarka Sampradaya, as Nimbarka, the founder of the tradition, declared that Radha and Krishna together constitute the absolute truth. Radha is the most important gopi in Raas (Special kind of dance) with Lord Krishna. Radha is often referred to as Rādhārānī or "Radhika" in speech, prefixed with the respectful term 'Srimati' by devout followers.  Gaudiya Vaishnavas, believe that, in fact Radha is the original source from whom Lakshmiji emanated.
Brooklyn Museum - Radha Pining for Her Beloved Page from a dated Rasikapriya Series
In the Vaishnava devotional or bhakti traditions of Hinduism that focus on Krishna, Radha is Krishna's friend and advisor. For some of the adherents of these traditions, her importance approaches or even exceeds that of Krishna. She is considered to be his original shakti, the supreme goddess in both the Nimbarka Sampradaya and following the advent of Chaitanya Mahaprabhu also within the Gaudiya Vaishnava tradition. Other gopis are usually considered to be her maidservants, with Radha having the prominent position of Krishna's favour. It has been mentioned in the narration of the Skanda Purana that out of a billion of gopis, 16,000 are prominent. Out of these, 108 are important, and out of 108, eight are principal. Out of eight gopis, Radha and Chandravali are chief, and out of these two Radha is superior. Between Radha and Rukmini, Radha is superior. All the gopis are reincarnation of goddess Lakshmi.
Her connection to Krishna is of two types: svakiya-rasa (married relationship) and parakiya-rasa (a relationship signified with eternal mental "love"). The Gaudiya tradition focuses upon parakiya-rasa as the highest form of love, wherein Radha and Krishna share thoughts even through separation. The love the gopis feel for Krishna is also described in this esoteric manner as the highest platform of spontaneous love of God, and not of a sexual nature.
Proponents of the Gaudiya and Nimbarka schools of Vaishnavism give the highly esoteric nature of Radha's relationship to Krishna as the reason why her story is not mentioned in detail in the other Puranic texts.
The Bengali saint Chaitanya Mahaprabhu (1486–1534) is believed by many (especially by the modern-day ISKCON movement) to be an incarnation of both Radha and Krishna, simultaneously in one form. Throughout his life, Chaitanya lived as a devotee of Vaishnava tradition, not openly claiming to be any form of Avatar, but in his biographies it is claimed that he revealed his divine form to some of his closer associates.
"Although the effulgence of the moon is brilliant initially at night, in the daytime it fades away. Similarly, although the lotus is beautiful during the daytime, at night it closes. But, O My friend, the face of My most dear Srimati Radharani is always bright and beautiful, both day and night. Therefore, to what can Her face be compared?" (Vidagdha-madhava 5.20)
"When Srimati Radharani smiles, waves of joy overtake Her cheeks, and Her arched eyebrows dance like the bow of Cupid. Her glance is so enchanting that it is like a dancing bumblebee, moving unsteadily due to intoxication. That bee has bitten the whorl of My heart." (Vidagdha-madhava 2.51)
"Just as Srimati Radharani is most dear to Krishna, Her bathing pond is similarly dear. Of all the gopis, She is the most beloved of the Lord." (Padma Purana)
"The transcendental goddess Srimati Radharani is the direct counterpart of Lord Sri Krishna. She is the central figure for all the goddesses of fortune. She possesses all attractiveness to attract the all-attractive Personality of Godhead. She is the primeval internal potency of the Lord." (Brihad-gautamiya-tantra)
"Among all persons, it is Sri Radha in whose company Lord Madhava is especially glorious, as She is especially glorious in His." (Rig-parisishta)
^Radha in Hinduism, the favourite mistress of Krishna. In devotional religion she represents the longing of the human soul for God: The Oxford Dictionary of Phrase and Fable, 2006, by ELIZABETH KNOWLES
^Kinsley, D. (2010). "Without Krsna There Is No Song". History of Religions12 (2): 149. doi:10.1086/462672. Retrieved 2008-05-03."Nimbarka seems to have been the first well-known religious leader to regard Radha as central to his cult (thirteenth century)"