In the movies, they show one of the chief recruitment methods of dangerous mind-control cults is to have pretty girls out there gaining the interest of young men. Jehovah's Witnesses do not intentionally use that method, but it happens. The young girls are often seen as pure and innocent. The young man sees the only way to have her is to join her religion.
If she's slightly bad and dates outside the religion, she typically won't "go all the way" and that makes the guy want her more.
She's violating JW rules by dating you. If she allows any touching or sexual contact of any kind, she's violating more rules. Eventually, she will feel guilty about it and drop you or go to the elders about her guilt or she may leave the JW's. Either way, you are making her near future a crazy one.
If you care for the girl, find out the truth about her religion and tell her. If she chooses her family and friends in the religion over you, try to understand it's all she knows and let her go.