Gender identity? That would be a series of brief conversations, as Isabelle honestly doesn't know much about it herself when she meets Cy. Growing up in a relatively conservative environment, Isabelle didn't get a lot of education on genderqueer identities. Being in the more progressive space the campus provides lets her explore more, wear pronoun badges at events, make queer friends, etc, but she still has a lot of hangups herself.

Cy discovering that she sometimes is a boy would be a partial accident related to something like him finding her packer or other boymoding tools. Isabelle would be comfortable enough to tell him that she likes a range of gender expression, and depending on how much she's figured out by then, would probably go on to elaborate a little on how she's genderfluid without using that exact word. Meanwhile Cy would decide early on in the conversation that he's just gonna indulge her regardless without thinking too hard about it or really trying to understand. He can take good care of his subs without understanding the why behind every what.