The Criminals, more commonly known as the Crims, were originally an Ugly clique founded by Zane, who was the leader. After Zane turned pretty, the Crims became a pretty clique and expanded to include more members.
Zane started the Crims with six friends in Uglyville. They strived to be as tricky as possible, eventually figuring out how to hoverboard to the Rusty Ruins. On one such outing, they met a boy named David who wasn't from the city. He told them all about his home far from the city's grasp, a hidden place called the Smoke. The Crims were excited to hear about such a place, believing it to be exactly what they'd been striving for, and promptly made plans to run away there. On the day of their departure, however, two of their number chickened out. Shay, and most surprisingly, their leader Zane.
Zane quietly underwent the Operation and became a Pretty, but he never forgot his guilt at bailing on his friends. He regretted not going with them and so restarted the Crims in New Pretty Town where it soon became one of the most popular Pretty cliques. Potential members had to be voted in unanimously and were required to have tricky Ugly stories to tell. By encouraging his clique to constantly think about their Ugly pasts and to "stay bubbly", Zane was unknowingly resisting the lesions. Although he recognized that something was wrong with Pretties, specifically their memories, Zane had no idea the lesions existed until he learned about them with brand-new member Tally.
The Smokies smuggled a cure for the lesions to Tally and a note explaining why, but she was too scared to try the experimental pills on her own. Zane on the other hand was thrilled at the existence of a cure and encouraged her to either have the pills herself or give them to him. Instead, she chose to split the pills between them. Fully cured, Zane took the antics of the Crims up a few notches and with their help, collapsed a floating ice rink as a message to the Smokies that the cure worked. Soon after, he and the Crims planned an escape to run away to the New Smoke, just like the original Crims did.
They were largely successful. Unfortunately, many members were recaptured by the city and turned into Cutters. Crims that remained, along with new Crims recruited by Peris, were useful in distributing the cure to hundreds of Pretties once it became more widely available. After the Diego War that killed Zane, many Crims became government officials in Tally's city.