Here are a few possibilities:
* Cal Hockley: The most obvious foil is Cal, Rose's fiancé. He embodies everything Rose rejects: wealth, privilege, and a life of societal expectations. He represents the life Rose is desperately trying to escape, making Jack, the opposite of Cal in every way, all the more appealing.
* Ruth DeWitt Bukater: Rose's mother, Ruth, is another potential foil. She represents the rigid social constraints of the time and pressures Rose to conform. Unlike Rose, who longs for freedom and passion, Ruth is trapped in a life of societal expectations and fear.
* The other passengers: The vast majority of the passengers on the Titanic, especially those from first class, are also foils to Rose. They represent the opulence and privilege that Rose ultimately rejects.
Ultimately, the best answer depends on what aspect of Rose's character you are focusing on. If you are looking at her yearning for freedom and love, Cal is the clear foil. If you are looking at her struggle against societal pressures, Ruth might be the better choice.
It's important to remember that a foil isn't necessarily a "bad guy." They simply highlight the protagonist's traits by being the opposite in some key ways.