Aika Sakuranomiya is the older sister of Maika Sakuranomiya. 

Aika is a cute and kind older sister who cares for her family and tends to worry about Maika Sakuranomiya's social life because of her scary eyes. Her care for her sister has been shown many times as she tries to help whenever she can.

The story centers on a girl named Maika who gets a job at a cafe where all the waitresses are given certain attributes — such as "tsundere" or "little sister" — to embody while serving customers. The manager asks Maika to be the "Do S" (extreme sadist) waitress, so she will have to adopt a dominant and aggressive persona.
 
(Source: Anime News Network)

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High schooler Maika Sakuranomiya can't seem to land a part-time job, so when an opportunity comes knocking, she's up for anything. Waitressing at a cute café—sounds perfect! The manager demands that each waitress adopt a quirk in their service? Not a problem. Maika's ready to act flirty, or hot-and-cold, or like a little sister, or…sadistic?! Oh. That might be a tough one for our cheery and caring heroine! The original manga behind the popular anime!
 (Source: Yen Press) 
 Note: Chapter count contains 8 newly written chapters (one per volume) and 1 extra chapter (vol.7: 番外編 ; Comic Fuz 2020-09-28)