I don't know if I can put this as sidekick, so I'm going to put it as costume for now.
THE SYMBIOTE
A
symbiote, in
Marvel Comics' fictional universe, is a living, sentient, alien organism that bonds with other living organisms in order to survive. Since it has no classifying name, it is referred to as a symbiote because of its
symbiotic relationships. Often symbiotes are called "living costumes" because of the way the amorphous creatures envelop their hosts. They have incredible adaptive attributes and quickly gain similar powers to the superhuman they bond to. They are extremely deadly and have murderous urges on a regular basis. Some symbiotes, like Venom and Carnage, are known to threaten to eat their victims' brains, among other organs (Venom has made regular threats to eat Spider-man's
spleen), and Venom has been hinted to have eaten some of the criminals he kills to sate the symbiote's lust for blood.
The symbiote enhances the physical attributes of its host, commonly increasing their strength and speed to superhuman levels as well as granting them a range of other powers such as transformation and mass-alteration.
The first appearance of a symbiote occurs in
Secret Wars #8 in which it bonds with Spider-Man. Eventually the symbiote attempted to permanently bind itself to the character, and had to be forcibly removed, after which time it bonded with
Eddie Brock to become
Venom. Since then numerous symbiotes have appeared, many of which are the offspring of the original Venom symbiote and possess similar or even greater powers. The second symbiote was the Carnage symbiote which bonded with Cletus Kasady to form
Carnage, whom then had its own child, which Venom named
Toxin.