Lou is the central mermaid character in the animated film Lu Over the Wall. She is a member of the merfolk, a sea-dwelling race that humans have long regarded with suspicion and fear. Lou lives with her father in the ocean near Hinashi Town. Her mother was killed in an accident involving a ship, an event that shapes her family's cautious attitude toward humans. Despite this tragedy, Lou harbors a genuine desire to befriend the people on the shore.

Lou possesses a childlike personality. She is curious, easily captivated by new experiences on the surface, and expresses joy freely through music and dance. She is innocent and trusting, approaching the world with an open heart. Her primary motivation is to connect with others and share the happiness she feels when she hears music. She is drawn to sound and rhythm, which trigger her unique ability to transform her tail into human legs whenever music plays. In this state, she can walk and dance alongside humans. She also has the power to control water and can compel those around her to dance through her singing.

Her role in the story is to serve as a catalyst for change in the life of the protagonist, Kai Ashimoto. She is the source of wonder and conflict that pushes Kai out of his loneliness. When Kai and his friends form a band, Lou joins them, and her performances quickly make her a local celebrity. This fame revives a struggling amusement park but also stirs up longstanding anti-merfolk sentiment in the town. She becomes the target of fear and blame when a misunderstanding leads to a crisis, and her father’s attempt to rescue her triggers a magical flood. Lou ultimately helps Kai and the townspeople resolve the disaster, proving that merfolk and humans can coexist peacefully.

Lou’s key relationships include her bond with Kai, who becomes her first human friend and later her love interest. She also forms friendships with Kai’s bandmates, Yuho and Kunio. Her father is deeply protective of her and is wary of humans, but he later supports her wish to bridge the two worlds.

Over the course of the film, Lou develops from a carefree, impulsive creature into a character who understands the complexity of human emotions and the consequences of her actions. She learns about trust, sacrifice, and the importance of communication. Her growth helps heal the rift between her kind and the townspeople, and she ultimately chooses to return to the sea while promising to see Kai again.

Lou’s notable abilities include transforming her fins into legs when exposed to music, controlling water, singing with enough power to make listeners dance involuntarily, and surviving underwater as a natural mermaid. However, she is vulnerable to direct sunlight, which can burn her.