Ravana's father, Vishrava, was a brahmin; his father was Pulastya, mind-born son of Brahma.
Vishrava had another wife, and with her, another son: Vaishravana.
Vaishravana practiced austerities for a thousand years, living on water, then air, then nothing.
Brahma offered him a boon.
"Make me a guardian of the world!" Vaishravana said.
Brahma nodded. "Indra guards the east, Varuna the west, Yama the south; you will guard the north. You will be Kubera, lord of yakshas, protector of wealth!"Kubera lived in Lanka, the golden city. The rakshasas had lived there long ago, but the devas had driven them away.
Inspired by: The Ramayana (Uttara Kanda: Sarga 3).
Notes: You can find out more about Kubera at Wikipedia.