Cruel Intentions 2

The film opens with Sebastian Valmont (Robin Dunne) conversing with his soon-to-be ex-principal, the principal's insistence on having Sebastian's permanent record relayed to his new school and thereby hampering his chance for a new start at Manchester Prep. Sebastian was a bad boy and a troublemaker in this school. Mostly he usually got in trouble with his teachers and principal. Initially, his principal was considering not to send his permanent record to his new school, but then Sebastian pulled a cruel stunt on his wife and made him and his wife a laughing stock in the community, he decided to send Sebastian's permanent record to his new school. Following his arrival in New York, Sebastian discovers the wealth of his new family; meeting Kathryn Merteuil (Amy Adams) for the first time and bettering her with piano and vocabulary. This leads to a confrontation between Kathryn and Sebastian whereby she states that she has a comfortable lifestyle and that he "better not interfere".
Sebastian later begins school. While waiting to see his new headmaster, he encounters Danielle Sherman (Sarah Thompson), who is, unknown to him, Headmaster Sherman's daughter. Luckily Sebastian switched permanent records before it was sent to the headmaster's office and thus,he can start over with a clean slate. A school assembly follows, showing Kathryn delivering a speech to her classmates, but being persistently interrupted by uncontrollable hiccups coming from a student, who then begins to choke on the gum that she was chewing in a bid to stop her hiccups. She is saved by the quick action of Danielle who performs the Heimlich maneuver, allowing the student to expel the gum, which ends up flying into Kathryn's hair. A meeting of a secret society of student elites presided by Kathryn takes place, deciding upon the fate of the new students. This leads them to Cherie, the student with the hiccups, as well as the discovery that Cherie's family is wealthier than that of Kathryn; this, and the events of the assembly, cause Kathryn to seek a vendetta against Cherie.
Sebastian, coming from a more humble upbringing, wishes to befriend his house staff. Doing so angers Kathryn, whose day is interrupted by not being able to contact her driver. This (along with Kathryn's jealousy of Sebastian) causes her to admit that she is unhappy with her life. Sebastian attempts to woo Danielle: first, by asking her for coffee at her work; then, later, conversing with her over the telephone. Eventually, this evolves into a relationship, but Kathryn, seeing this, uses it as a way to get back at Sebastian. She first tries to tempt him away from Danielle by luring him with identical twins, who confide to Sebastian that Danielle is the only virgin at Manchester.
Kathryn's attempt to sabotage Cherie backfires, as Kathryn's mother tells her to become best friends with Cherie, in an attempt to encourage Cherie's mother to donate a large amount of money to the school. In the end, Sebastian stays with Danielle; he professes his love for her, only to find that she does not reciprocate. It turns that Danielle is actually working alongside Kathryn in a secret plan to dupe Sebastian. Defeated by Kathryn's manipulation, Sebastian states "if you can't beat them, join them", thus leading to a threesome with Danielle and Kathryn, followed by an alliance of the three to dominate and manipulate others. In the last scene, Cherie is seen riding her bike, which is run over by Sebastian's car. Sebastian offers to give her a ride, and has sex with her. Kathryn and Danielle are seen, in the front of the car, pleased with the results.

Question: Who is working alongside Kathryn in a secret plan?
Answer: Danielle.