Ravana is a powerful king of demons from Hindu mythology and is often depicted as having 10 or more heads - he is famous for kidnapping the wife of Rama, although he is seen primarily as the antagonist of the story he has had some followers and devotees in the Hindu faith who see him as a more morally flexible being than his more commonly seen evil incarnation. Ravana is also the main antagonist from Sita Sings the Blues.
In the story of the Ramayana, he kidnapped Sita, wife of Rama, and shut her away in a tower so he can take her as his wife. Fortunately, Rama came to Sita's rescue and, in a fierce battle, killed Ravana with a poisoned dart.
Ravana is by far one of the most well-known and popular villains in all of Hindu mythology and has gained a great many outings in media outside his native mythology, often keeping as an antagonist.
Ravana is also seen as an incarnation of the demon Kali and as such one of many avatars that the great spirit of evil spreads its influence, this is mirrored in the Ayyavazhi mythology where Ravana is seen as an avatar of Kroni, who is the supreme spirit of evil within that setting.