Isabelle, also known by her nickname Izzy, is a supporting character in To The Moon. She, like River, has Asperger's syndrome, though she was diagnosed early in life and was taught to act "normal". Johnny became acquainted with her through his best friend, Nicolas, and asked for her help to manage River's condition.
Isabelle only appears in Johnny's memories and her appearance is muddled by static. She seems to have short brown hair in a bob cut. Her clothing looks to be a long-sleeved green dress with white collar and white ankle-length skirt.
Isabelle is an Aspie (a friendly term meaning someone with Asperger's), but she appears like a typical person as she was diagnosed and underwent treatment at a young age. In the library scene, she described herself as an actress who had to fake being normal at every moment, and that she envied River, who never did and kept being herself.
Isabelle is a kind and understanding person. These positive traits are shown in the same scene when she empathized with Johnny's struggle in coping with River's condition. However, when River fell terminally ill and Johnny told Isabelle he didn't want to risk River refusing her treatment so he could finish building the cliff house, she criticized Johnny for being arrogant and thinking he knew what was best for her. She believed that River had a right to choose and sided with her.