Alright, since my first pick wolverine got stolen, and I wasn't allowed to use the symbiote on The Punisher, we'll go with the symbiote itself.
"The creature is a sentient alien with a gooey, almost liquid-like form. It requires a host, usually human, to bond around for its survival. In return the Venom creature gives its host enhanced powers. In effect, when the Venom Symbiote bonds with a human to form a super-villain, that new dual-life form itself is also often called Venom. The symbiote, with or without a host, is an archenemy of Spider-Man, who was the first host it merged with before its evil motives were clear.The symbiote acts as an evil double of the host after it bonds with its host. After Spider-Man rejected it, the symbiote went on to merge with other hosts and so began its popular reign as the villain known as Venom."