Rukmini's is an incarnation of the goddess of prosperity, Lakshmi.
Bhagavata Purana narrates that Rukmini once heard about Krishna and his heroic deeds, such as slaying the tyrant king Kamsa, and opposing the evil king Jarasandha.
Rukmini's parents rejoiced and gave their permission, but Rukmi – who was an ally of Jarasandha – strongly opposed it.
Bhishmaka agreed, and a distressed Rukmini immediately sent for a trusted Brahman and asked him to deliver a message to Krishna.
At the palace, Rukmini had lost all hope, but the messenger turned up and informed that Krishna had accepted her request.
Rukmini challenged Krishna to a fight, but was easily overpowered by the former.
Rukmini begged Krishna to spare her brother's life, and the latter agreed.