My Brother's Contract Girlfriend is My Mermaid Bride - Episode 6

The protagonist is heartbroken after being cheated on. When her brother's new girlfriend, a woman who seems to be using him for fame, moves in, the protagonist's brother makes a deal to have her surveilled 24/7. As the protagonist studies the girlfriend, she notices strange scales on her skin. The key turn comes when the girlfriend, hearing the protagonist's brother declare he dislikes scales, quietly concludes that this means he hates mermaids.