Asukasu

Asukasu is the family ship between Kasumi Toyama and Asuka Toyama from the BanG Dream! fandom.

Canon
Kasumi and Asuka are sisters with a one-year difference; Kasumi being the older sister, while Asuka is the younger one. Kasumi calls Asuka with the nickname "A-chan", while Asuka calls Kasumi simply "Onee-chan". The two have contrasting personalities, as Kasumi is more extroverted, energetic and loud, whereas Asuka is more rational, introverted and sarcastic. Despite this Asuka cares for Kasumi and is bad at showing her affection for her.

Most of their material comes from the first season of the anime, where Kasumi is shown to being affectionate quite towards Asuka. In one scene, Kasumi wakes up Asuka hugging her and shaking her around while she's still under her bed covers. In another, after Asuka tells Kasumi some encouraging words, she motions her to come closer and gives her a kiss on the cheek. Surprised, Asuka runs out embarrassed and calls out to their mom while yelling how weird Kasumi is being.

The two also used to attend the same school, with Asuka being part of Hanasakigawa's Middle School branch. However, she changed schools to Haneoka Girls Academy in high school.

Fanon
Asukasu is very much a rarepair. It's rarley shipped by anyone in both the Eastern and Western fandoms. Most peoples' disdain for the ship comes from not only because it is incestous, but because they also prefer other ships for both girls (such as Kasuari for Kasumi, and Asurokka for Asuka). They sometimes can be seen interacting in fanfiction as family.

Fandom
PIXIV
 * Toyama Shimai (Toyama Sisters) tag on Pixiv

Trivia

 * Kasumi and Asuka's voice actresses, Aimi and Ozaki Yuka, used to be hosts for the BanG Dream!-related TV show Bandori! TV, and had a corner dedicated to where Aimi taught Yuka how to play guitar. However, she later quit as the co-host and was then replaced by Maruyama Aya's voice actress, Maeshima Ami on the 22nd episode.
 * In the 161th 4-panel comic, Asuka is shown to be able to understand Rokka's words due being able to understand what Kasumi says most of the time (in the comic, she is called a "Kasumi Language Expert".)