Cassie is Monica and Ross's cousin, and the titular character in "The One With Ross And Monica's Cousin". A very beautiful woman, she causes Chandler, Ross and even Phoebe to be attracted to her when she comes to visit, leading to tense situations as they are her cousin's fiancé, cousin and another woman respectively.
There is no reference to her family name. If Cassie is from Jack's side of the family her last name is probably "Geller". If she is from Judy's side of the family, then her last name is probably different.
When she was a child, Cassie used to carry a Barbie everywhere. One summer, her parents along with Jack and Judy rented a beach house with their children. During those holidays, Ross pinned her down and tickled her until she cried. She also got many freckles on her chest.
The last time she and Ross saw each other was when Cassie was around 7-8 years old. Ross was setting up his tent in line to see Return of the Jedi, but Cassie pretended not to see him out of embarrassment.
Cassie returns many years later to stay a few days with her cousins before Monica's wedding. Now at 25 years old, she has grown into an incredibly attractive woman, which causes Chandler to be instantly captivated by her when he sees her getting ready to have a lunch with Monica. As he is unable to stop staring at her, Cassie has to stay with Ross.
However, the situation repeats as Ross gets shocked at her beauty at the very moment of opening the door, and can't help but stare at her too; though Cassie believes he's just imitating Chandler. They talk about the last time they met and the summer they spent at the beach, while Ross secretly tries to control his desires due to the fact that they are cousins.
They later sit on the couch to see Logan's Run and drink wine. Ross mistakenly interprets the romantic atmosphere as a sign that his feelings are reciprocated and tries to kiss Cassie, but she dodges the kiss and in disgust and anger asks him what is he doing. Ross is left speechless by the negative reaction, and after a long and tense silence, he only manages to say that he hasn't had sex in a very long time, which horrifies Cassie and she leaves the sofa.
Cassie later stays with Phoebe, but once again, after Phoebe sees her getting out of the shower, she experiences the same attraction that Chandler and Ross felt, and stares at her.