Ahn Seo-hyun

Bomi is 12 years old. She wants her own room so badly. Her sister, Eonju, whom she shares the room with keeps sneaking a look at her diary. She studied really hard to get first place as her mom promised to get her own room if she did but her mom keeps denying the promise.

Ho Goo-hee, a high schooler and self-proclaimed pushover, discovers a new application on her phone that offers to take revenge on anyone whose name she writes in it. (wikipedia)