Younger sisters(aka imouto) are essence in many animes/mangas/VN. They deserve a thread to talk about. Let's talk about any younger sisters, not necessarily one of main character's. We can talk about younger sisters in general or specific characters we like.
These all count.
-younger sister (ex. Kokuto Azaka)
-stepsister (ex. Tohno Akiha)
-half-sister (ex. Illyasviel von Einzbern)
-younger of the twin (ex. Fujibayashi Ryou)
These don't(in my opinion).
-cousin (ex. Kobayakawa Yutaka)
-clone (ex. Lycoris)
I like Ryougi Shiki and Kokuto Azaka from Kara no Kyoukai. It is not very well known but Shiki actually does have an older brother(mentioned in chapter 2 & 4). I like Shiki just because she is cool(love her eyes). I like Azaka too because of her love toward her brother, Mikiya. It is really fun to read about it in the novel (sad that animated film version ruined it).