Vajradhara is the primordial buddha, the dharmakaya buddha. Vajradhara, depicted as dark blue in color, expresses the quintessence of buddhahood itself. Vajradhara represents the essence of the historical Buddha's realization of enlightenment.
Historically, Prince Siddhartha attained enlightenment under the bodhi tree in Bodhgaya over 2500 years ago and then manifested as the Buddha. According to Buddhist cosmology, he was the Fourth Historic Buddha of this fortunate eon. Prince Siddhartha's achievement of enlightenment, the realization, or wisdom of enlightenment itself, is called the dharmakaya, the body of truth. When he expresses that realization through subtle symbols, his realization is called the sambhogakaya, the body of enjoyment. When such realization manifested in more accessible or physical form for all sentient beings as the historical Shakyamuni Buddha, it is called the nirmanakaya, the body of manifestation.
The dharmakaya, synonymous with Vajradhara Buddha, is the source of all the manifestations of enlightenment. Vajradhara is central to the Kagyu lineage because Tilopa received the vajrayana teachings directly from Vajradhara, the dharmakaya buddha. Thus, the Kagyu lineage originated from the very nature of buddhahood.
That realization of the nature was in turn transmitted within the Golden Rosary by Tilopa.