Ningyohime 2001 - Aqua

Jinae and Jadae are twins. Ever since they were together when they were little, Jadae is always the one who is praised by the adults, being outgoing and older.
Whenever an accident occurs, the one to be scolded was always Jinae.
To Jinae, everyone likes Jadae better than Jinae, ranging from her dad, uncle, aunt and the sensei at school. Due to this, she hates adults.
Then, she met a cute boy at the beach! Also, He's the first one who's willing to listen to Jinae's story about mermaids.