Age in Narcissu: 22
Age in Narcissu Side 2nd: 15
Setsumi has a childlike physique. She has a taciturn and stoic kind of personality. She loves cars and is knowledgeable about them. Because of her terminal illness, she has no friends and is always seen alone. After many years of receiving treatment, she became totally desperate about everything - life or time, or each day passing doesn't matter to her anymore. It is just like she stated once: "My time came to a halt". Everything is very painful to her because she was left behind alone as the world around her continued to move on.
In Narcissu, she met Atou Yuu, a new resident of the hospital's 7th floor, the hospice, and for a short moment did she come to know something like hope. <hidden by spoiler settings>They escaped together and headed to Awajishima, to watch the beautiful blooming narcissus. Along the way, they grew attached to each other and the girl found herself emotional again, happy with her new skirt, blushing... At the end of their journey, she asked for a bikini, and after a long day of playing at the beach, she went into the sea and ended her very life.
In Narcissu Side 2nd, when she was still an outpatient at her age of 15, she met a resident of the hospital's 7th floor, one of hospice's patients, called Himeko. Their bond grew quickly as Himeko and Setsumi enjoyed their summer days together.
[Edited from Wikipedia & Neechin.net]