It's about a guy Transmigration into a book as a small mermaid the size of hand, he wants to help the original ML (not sure if it was the OigML or orginal villain) because its his favourite character, he can return to his original size and human form unlike the other mermaids in the story, and he even has a space that grows food, I also remember that the MC wasn't your average juo as he is a vary capable fighter. And everyone is jealous of the ML because his relationship with his mermaid is better than everyone else,
I know this one, I'll return in a few minutes Update: Transmigrated Into the Violent Boss’ Little Mermaid