Squid Girl is a girl from the ocean who possesses squid-like tentacles for hair. She vows to invade mankind as revenge for its pollution of the ocean. Her first task is to make the Lemon Beach House snack bar, owned by the Aizawa siblings, her base of operations. However, when she accidentally breaks a hole in their wall trying to swat a mosquito, she is forced to work as a waitress to pay off the damages. Thus, Squid Girl begins her life on the surface, learning new things and encountering many curious visitors to the beach house.

Squid Girl (イカ娘 Ika Musume)
The main protagonist of the series. Squid Girl is a girl who comes from the ocean. She has plans to invade humanity as punishment for the pollution done to the sea. However, after causing damage to a beach house (which she planned to use as a base) while trying to swat a mosquito, she is forced to work as a waitress to pay for the damages and also sleeps at the Aizawa residence. Like a squid, she has ten fully controllable blue tentacles that protrude from the top of her head like hair that can regenerate if they are cut off. Squid Girl can spew squid ink from her mouth. She also possesses numerous abilities from various species of squid, such as glowing in the dark like the firefly squid, moving the little flaps on her hat (which are actually her fins) or changing her weight at will in order to better control her tentacles to do stuff like deliver a powerful smack, swim faster, wrap them around a foe and throw them into the air, carry many plates, and draw well. According to her, she will die if she takes her hat off, which is made apparent when trying to force it off causes her life to flash before her eyes.
She enjoys shrimp and greatly fears natural predators of squids like sharks and killer whales (to the point that even swimming floats shaped like killer whales terrify her). Although generally immature and often getting herself into mischief, she is a fast learner, able to master complex things such as math and other languages in a short period of time (although this does not change that fact that many of the other characters are smarter than her). She almost always ends her sentences with "de geso" (でゲソ?, lit. "squid tentacles") as a copula and emphasizes "ika" in sentences, which is reflected by how it's written (e.g.:ja na ika (～じゃなイカ?, lit. isn't it?)). In the English dub, she incorporates various squid-based puns into her dialogue.

Mini Squid Girl (ミニイカ娘 Mini Ika Musume)
A miniature version of Squid Girl that appears in people's dreams (mostly Sanae's dreams). Like Squid Girl, she has a strong appetite for shrimp, though her vocabulary consists of only one word, which is 'geso' in the original version and 'squid' in the English dub. A Mini Squid Girl can purportedly live for up to 150 years.