Obsidian was a Gem whom fought Homeworld thousands of years before the events of the series. After the Homeworld forces retreated, he believed that homeworld gems should all be shattered. He attempted to persuade his allies to invade Homeworld, but they declined, since they believe Homeworld would attack the Earth a second time out of retaliation.
Unlike most villains in Steven Universe, he adores humans and the Earth, and would never hurt an innocent human. He does, however, believe that Homeworld gems (including the Diamonds) should be destroyed. When faced with Homeworld gems, he shows no mercy, as he views them as filth. He also believes that the Diamonds are a plight to the cosmos and enjoyed watching Rose Quartz shatter Pink Diamond.
He manages to create other forms of life using the same cooling process but in his case the obsidian that creates becomes alive.
His weapons vary in shape, his weapon is a kind of sword like that of Steven but similar has a katana.