Yumi is part of a diamond smuggling operation.
After Kenji escaped, Rhys and Ben used the cameras Rhys had placed to find the person who came to collect the diamonds. Unfortunately, Justine noticed that she had a diplomatic pouch with her. They figured out the woman worked for Kohana Takashi, the Japanese Counsel General, but the FBI decided not to pursue the lead because they didn't want to create an international incident. Rhys and Ben then suggested they investigate and then once they had what they needed, they could leak enough information to the press to force an arrest, getting the arrest without creating an incident. Their way in was through Kohana's son, Jesse, who is into all things high-end. They planned an exclusive dinner party and then invited all Jesse's friends, to make him desperate to attend. Once Jesse figured out all his friends had been invited, he came to Rhys directly to ask for an invitation.
At the party, Rhys charmed Kohana as she drank. At the end of the evening, Ben served her fugu to give her a taste of what they could offer. Rhys let her know that their suppliers could get whatever they needed. Jesse stepped in and, seeing that his mom was drunk, said he was going to call a car to take her back to the consulate. After she was gone, Rhys and Ben were shocked when Jesse came back and told them he was the diamond broker. He told them to get rid of the crowd and tell him what they did with Kenji or Yumi would kill them.
Once everyone was gone, Jesse held a gun on them and they suggested they become his next front for his diamond smuggling. In order to save their lives, Rhys offered him the $3 million he'd stolen from Tommy. Because it was in Rhys's car, he sent Ben to get it after slicing Rhys with a knife, the same knife used to carve the fugu. ("The Dining Hall")
When Alice ended up taking the $3 million to save Tommy's life, Ben went to get the same amount of money from Margot to save Rhys. Meanwhile, Rhys started to show signs of damage from the toxin. In order to save his own life, Rhys talked his way into a deal with Jesse and Yumi. Yumi then injected him with epinephrine to save his life and then they agreed to a deal. They formed a new partnership to continue smuggling diamonds through Kenji's restaurant.
Ben and Rhys went to Justine with their new plan and she asked them to figure out which of them was truly the leader, so they could figure out which one had the power. When they re-opened the restaurant and signed for the fish order with the diamonds, they were shocked to find that the diamonds had already been removed from the fish.
When they told Justine about the missing diamonds, she suggested that either Yumi or Jesse had taken them. She wanted to turn them on each other so they could hasten the breakdown of their relationship. She talked to Jesse about what happened the night she took his mom home and Ben took compromising pictures of the two of them. Rhys then took the picture to Yumi and showed them to her. Ben took pictures of the two of them together and took those and showed them to Jesse. They pitted the two of them against each other. When they went to the restaurant to get the diamonds, they made each of them think the other one had taken the diamonds. They got each of them to confess to being the ringleader. Then Justine and the FBI came in and arrested both of them. They were then extradited to Japan. ("The Family Way")
She and Jesse had a romantic relationship as well as a professional. Ben and Rhys exploited this to get them to betray each other. ("The Family Way")
She is involved in the business of smuggling diamonds into the United States. She also worked in some capacity for Kohana Takashi.