Riot is a spawn of the Venom symbiote.
History in the comics
Trevor Cole was a volunteer/mercenary for the sinister Life Foundation, an organization that was preparing for the M.A.D. (Mutally Assured Destruction) fallout of the Cold War and sought to provide a comfortable life to their wealthy clients after the impending nuclear holocaust. Trevor was picked from the organization's security force made up of cops, soldiers, and mercenaries to take part in an experiment. Venom went through a procedure where multiple "offspring" were crated from him - thus Riot, the product of a symbiote spawn and Trevor was born. Unlike the other symbiote hosts, Trevor was extremely agressive and unsympathetic, far worse than his "siblings" (with the only exception being Carnage).
Trevor's lack of control over the alien inside him kept resulting in violence, so he and the other Life Foundation symbiotes kidnapped Eddie Brock in order to learn control over their powers. During Venom's time as Lethal Protector of San Francisco, Riot and his "siblings" were defeated by Venom and Spider-Man. Trevor's symbiote was artificially aged to dust and his body was caught in a massive explosion.
It was later revealed that Riot and the other symbiotes had survived due to efforts by the Life Foundation. They broke Eddie Brock out of prison and kidnapped him in a last ditch effort to communicate with their alien "others." Unfortunately Eddie refused to aid them, due to Carnage's example that his "children" became psychopathic killers. Scream killed Trevor with a sonic knife after killing Leslie (Agony) having snapped from his schizophrenia and her symbiote's influnence, believing that the symbiotes and any who host them were evil. Trevor's death assisted Donna in misleading the others into thinking Eddie Brock was picking them off.
Riot possesses all the powers of Venom such as wall crawling, super strength, agility, and can naturally fire webbing from his wrists.
Riot also has a healing factor and can camouflage himself.
Trevor could morph the symbiote into various weapons such as hammers and maces from his arms.