Greg Sherman is a con artist that runs a boiler room operating the Inverted Pyramid. This is where you call 20 people and tell 10 the stock's going up, and tell the other 10 it's going down. Whichever one you get right, you call those 10 people back and do it all over again. Once you have done that you have convinced 5 people you can predict the market. However, he does this on a much larger scale. In each city, he sets up a boiler room, hires a bunch of hot shots, and they call a thousand people at a time.
He is not just any con man; he is a direct descendant of the Yellow Kid who was quite possibly the greatest grifter of all time. Between 1900 and 1940, he swindled people out of more than $10 million; an incredibly high sum in todays money. His father was the Springfield Wonder and through him he knew Titanic Thompson and Wildfire John. They taught him every con in the book mostly because they invented most of them. His father always told him, if they're not smart enough to hold onto their money, then they don't deserve it. Due to his upbringing, he works without scruples and without conscience. There is no con, no play, no action that the Leverage team can pull that he hasn't done a thousand times. He knows every shill, every player in the country.
He calls himself "The Mako" after the shark of the same name due to him always moving and if he stops he dies. He reads people very well. Sophie said he reads people better than anyone else she knows. He picked up on Nate's and her relationship down to just their body language and the looks Nate gave her.
As the Leverage team know they cannot con him, they decided to rob him. They set up what looks to be a con involving cocoa futures and controlling the cocoa market. However, this is just a ruse for him to reveal where he keeps his money so that they can steal it. As his grandfather invented the "big store" they knew he wouldn't fall for the con, but because he was so focused on how they were trying to con him he didn't realize it was all part of the plan to divert his attention from them robbing him, with help from over a dozen of his victims (including a school teacher who brought him to the attention of the Leverage team).
However, by the end, it is revealed that Sherman had figured out their scheme and managed to manipulate things so that he could exploit the cocoa market to make millions (oblivious to the fact that the Leverage team had already moved to steal his money) and would walk away, to wait for his scheme to pay off and spend the rest of his days in a country with no extradition treaties with the United States, to bask in the infamy of his crime. However, his triumph was short-lived as the FBI suddenly arrived to arrest him. As he is led away (to face 47 counts of security fraud, conspiracy, and market manipulation), the Leverage team and his victims all blurt out alternative (and less scary) nicknames that they claim Sherman used.
- A running gag throughout the episode is everyone "forgetting" what his nickname is, using any other type of fish they could think of at the time like Blowfish.