We were introduced to Miss Gillies when she took her class to learn about plants in one of the greenhouse cars at the same time that Layton got taken there by Roche.
During LJ Folgers trial she was the representative of second class who decided on the guilty verdict, the verdict would be overturned by Melanie out of fear LJ would reveal the true nature of the drawers, much to the representatives frustration.
Miles is one of Miss Gillies' students. On his last day, before he is taken to the Engine as "Mr. Wilford's" new engineer, he is accompanied by Bennett. Miss Gillies gives him an orange with some sort of substance inside that causes him to be sick, therefore allowing him to be taken to the 2nd Class Clinic Car, where Josie was disguised as a nurse. That allowed the rebel army to use Miles' presence in the engine, which later led to the appearance of LJ Folger in the Engine Car. Miss Gillies can be seen trying to get Bennett's attention while Josie is talking to Miles, trying to flirt with him and asking him numerous questions about his job. Later, when Josie finishes talking to Miles and leaves the room, Bennett realizes something is wrong and tries to follow her - with no result. Miss Gillies can be seen making eye contact with the doctor afterwards, clearly indicating that she was in fact cooperating with the rebel army, although her motives were unknown and she's suspected to be one of the "good people" of 2nd Class, much like the doctor, who decided to help the rebel army, even though they had nothing to gain from it.
In the Tailies' Revolution, Miss Gillies visits Martin Colvin under the pretense of talking about tutoring for his children. With neither wanting the Folgers and Commander Grey in charge, Martin slips her a gun that she hides in the sub-train for Andre Layton and Roche to find.[1] Later when Andre Layton have full control of the train and is making an announcement, she is seen listening among the crowd.
In A Single Trade, she is seen among the partying people and also watching Melanie's connecting to the first balloon.
In Keep Hope Alive, her class presents a mural that they have painted, to support Melanie.
In The Tortoise and the Hare, six months after the departure of the Pirate Train, Doctor Pelton pulls Winnie out of class to deliver a secret message to Ruth Wardell. Apparently a member of the continuing resistance against Wilford, Miss Gillies simply acknowledges the action with a nod and continues on teaching.
In The Last to Go, she helps LJ do the make up and get ready for the wedding.
When she needs to make the choice between staying on Snowpiercer or going to New Eden on Big Alice. Miss Gillies chooses to stay on Snowpiercer, standing holding hands with Doctor Pelton and listening to Miss Audrey singing in the Night Car after the separation.