Ika Musume is the central figure of her narrative, a creature who emerges from the ocean depths with a clear but ill-fated mission. Her background is rooted in the sea, which she considers her true home and a domain that has been profoundly harmed by human pollution. Driven by a sense of offense at this environmental damage, she decides to swim to the shore and launch an invasion of the surface world, intending to punish humanity for its transgressions against the ocean. Her plan, however, is derailed almost immediately. She chooses a seaside restaurant called Lemon beach house as her staging ground for conquest, but in a clumsy accident, she damages the building's wall. Unable to pay for the repairs, she finds herself forced to work as a waitress at the very place she intended to conquer, setting the stage for her unexpected life on land.

In terms of personality, Ika Musume is a complex blend of ambition and naivety. She proudly declares herself a messenger of the sea and a fearsome invader, yet her actions rarely align with this villainous self-image. She possesses a natural and straightforward disposition, often misunderstanding common human customs and objects due to her oceanic upbringing, which leads to comical situations. A key part of her speech is her distinctive verbal tic, which is often translated as ending her sentences with a phrase like...is what I would say or with various squid-related puns in different language versions. Despite her stated goal of conquest, she is fundamentally good-natured and unable to commit genuinely malicious acts. She has a powerful, almost desperate love for shrimp, and displays an intense, paralyzing fear of her natural predators, such as sharks and orcas, even becoming terrified by inflatable toys shaped like them. She is also a fast learner, capable of mastering new skills, languages, and complex academic subjects in an incredibly short time, even as her common sense remains questionable. Her initial motivation to scare and subjugate humans slowly becomes more complicated as she begins to enjoy her new life and the company of the people around her.

Ika Musume's role in her story is as the catalyst for both chaos and connection. Her presence at Lemon beach house is the primary source of the narrative's comedy and conflict. While she constantly announces her intention to invade and conquer, her daily reality involves serving customers, cleaning, and becoming an integral, if disruptive, part of the seaside community. Her key relationships are central to her experience. She lives and works alongside the Aizawa siblings, who run the restaurant. She shares a vitriolic but deep friendship with Eiko, with whom she constantly bickers and competes. The older sister, Chizuru, is a figure of quiet but immense authority; her terrifying smile and calm demeanor can instantly quell Ika Musume's rebellious outbursts, keeping her in line. The youngest sibling, Takeru, looks up to her like an older sister, and she often finds herself playing and bonding with him. Other important figures include Sanae, a girl whose obsessive adoration for Ika Musume borders on harassment, and Nagisa, a part-time worker who is one of the few characters to genuinely fear Ika Musume's invasion plans, an ironic situation that Ika Musume finds amusing and enjoys exploiting.

Regarding her development, Ika Musume begins as an overconfident invader who sees all humans as enemies. Over time, her experiences and relationships cause this worldview to soften considerably. Her supposed mission of conquest becomes more of a familiar routine or a hollow boast, as she grows genuinely attached to the Aizawa family and her other human friends. Despite her claims, she begins to participate in human society, displaying a sense of responsibility and even a willingness to protect those she has come to care about. Her journey is one of finding a place where she belongs, transforming from a lonely creature of the deep to an accepted, if unusual, member of a found family on the shore. In the conclusion of the original manga, after finally repaying her debt, she deliberately damages the restaurant's wall again, giving herself a reason to stay, an act that fully reveals her attachment and cements her place among the humans.

Ika Musume is also defined by her notable abilities, which are all derived from her squid physiology. Her most prominent feature is the ten long, blue tentacles that grow from her head like hair, which are fully prehensile and incredibly strong, capable of lifting heavy objects or even punching holes in walls. These tentacles can also regenerate quickly if severed. She can produce a high-quality, non-toxic ink from her mouth, which is actually useful for cooking. Her body can glow in the dark, mimicking the bioluminescence of a firefly squid. The white, squid-shaped cap on her head is not an accessory but a vital part of her body; removing it would be fatal. She wears thick bracelets on her wrists that allow her to manipulate her own weight, shifting from being light enough to be carried away by the wind to being as heavy as several tons, which helps anchor her when using her tentacles. Despite her many powers, she cannot swim effectively without using her tentacles, and prolonged time away from the ocean can cause her abilities to weaken.