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Stephen "Steve" O'Toole was a terran who worked as part of the security detail for crime lord Ethan Stewart on the world of Dead Man's Rock. He called his employer "boss". Stewart's ex-mercenary partner, Rosemary Dahl, had apparently caught his eye, O'Toole considering her to be "a hot little number" – an assessment that he didn't hide from Stewart.
He had killed more than once.
In 2503, when said "hot little number" and fellow fugitive Jake Ramsey arrived at Stewart's residence looking for sanctuary, O'Toole was oblivious to their actions when they began their escape, as they had discovered that Stewart intended to "sell" them to his patron, Valerian Mengsk. O'Toole conversed with his employer normally, even while Stewart was held at gunpoint by the pair, as O'Toole had not realized this. The pair eventually escaped, O'Toole getting caught up in a psychic field created by Ramsey. 
Immediately after the escape, Valerian Mengsk contacted the Stewart compound, but the place was in chaos due to Stewart's severe injuries and the after-effects of the psychic phenomenon. O'Toole claimed to be in charge, and gave a report of the fiasco to Valerian. Shortly afterward, the zerg invaded the planet, destroying Stewart's compound. O'Toole was presumably among the list of casualties.