She was the daughter of Nagnajita of Kosala, the king Nagnajit had taken a vow of giving his daughter's hand in marriage to whoever tames his 7 bulls and puts noose on them successfully. Many had tried but failed.
It is described that there are three aspects of the Supreme that must... heard about this he went to Kosala, he was greeted by the king and worshipped by him, When krishna expressed his intent to marry his daughter. He mentioned the task that Krishna has to complete before having her hand in marriage. Krishna assumes 7 forms and tames all of them at once, thus takes her hand in marriage.