Kumbhakarna was the brother of King Ravana of Lanka. Ravana, Kumbhakarna, Vibhishana were the sons of a sage named Vishrava, born to a Rakshasa woman named Kaikasi. They had a sister named Shurpanakha. Kubera is their half-brother, born to another wife of Vishrava.
Along with his brother Ravana, Kumbhakarna indulged in rigid austerities with the intention of pleasing Lord Brahma. When Brahma appeared before him, he wanted the boon of immortality (Nityatva), but the Goddess Saraswati caused him to mispronounce the word as sleepiness (Nitratva). Appalled, he pleaded with Brahma to rescind the boon, which had really become a curse. Brahma could not take back his boon, but he modified it such that Kumbhakarna would be asleep for six months and then awake for the other six. (Some sources say that Kumbhakarna was given only one day of wakefulness every six months).
He was a giant in stature, and consumed huge amounts of food when he was awake. One of the most powerful warriors on Ravana’s side, he was also very knowledgeable about proper conduct. Like his brother Vibhishana, he counselled Ravana to return Sita to Rama to avert the war, but unlike Vibhishana, he did not abandon Ravana when his advice was rejected.
He fought valiantly against the Vanara army of Rama, causing great damage. Ultimately, he was slain by Laxmana with the help of a divine missile. Though he fought on the side of evil, his devotion to Ravana and his loyalty to his kinsmen is highly praised, and is contrasted favorably with the "treachery" of his brother Vibhishana, who fought on Rama’s side