Wendy is a Class A member, belonging to the Human Resources division of Team Rocket. Her job is discussing potential new Rocket members with Giovanni. When Jessie, James, and Meowth's memberships expire, they decide to 're-enlist' with a local recruiter (apparently Johto has a lack of Rockets). As Wendy processes their information, she flies into a rage.
Apparently, Wendy and Jessie had exercised at a Rocket Gym together in the past. After their workout, they both enjoyed fruit smoothies, but Jessie left the tab with Wendy. The green-haired HR Rocket is still furious to this day. As the application fees are deducted from Jessie and James' signing bonuses, Wendy adds in the cost of one fruit smoothie (Jessie doesn't remember the infamous event, claiming that she doesn't even like fruit smoothies - "Give me a double-espresso half-caff mocha latte with extra foam any day!")
Giovanni allows the three Rocket failures to re-enter his "evil organization" but instructs Wendy to "Work them hard until their debt is paid off. On second thought, just work them hard."