Wendy works in human resources for Team Rocket, and holds a grudge against Jessie because of something that happened years earlier. One day, they were working out together and afterwards got smoothies at the Rocket Café. Jessie left Wendy to pay for them and said "I'll pay you back someday." While Jessie forgot this over time, Wendy never did, and thus holds a constant grudge against her. Wendy got her "revenge" when she added the price of her smoothie to the trio's Team Rocket bill. Unbeknownst to her, Jessie didn't pay any attention to it. Wendy has direct contact to Giovanni and she refers to him as "master."
So she obviously has a temper, and that story may be what causes it. Keep in mind Team Rocket never wins, so the job is probably depressing for her. However, the definate answer lies among the anime writers, so yeah.