Estimated age: 13-14. A girl who came from the deep sea to the surface to invade humanity for polluting the ocean. She chose the beach house "Lemon" as her first base of invasion simply because it was closest to the sea, but failed miserably. Now, she is forced to work there as a low-wage employee to pay for the wall she broke. She carries out tasks assigned by the Aizawa sisters but easily gets distracted by anything interesting. She is highly curious and prone to causing trouble. She always wears a white school swimsuit under her white dress. The blue bracelets on her wrists allow her to control her weight, which she uses to adjust the power of her tentacles. The white hat-like top of her head resembles a squid's mantle and is part of her body; she will die if it is removed. The fins on the side of the hat move at her will and function as weapons against enemies. She recognizes anyone with these fins as a fellow squid. Her ten blue tentacles, integrated with her hair, are extremely dexterous and sharp enough to cut through objects. They are elastic and possess regenerative abilities, growing back by the next day if severed. Because she handles almost everything with her tentacles, her human hands and feet are clumsy. She can spit squid ink from her mouth and emit light from her whole body like a firefly squid. She always ends her sentences with tics like "~de geso" or "~ika?" (e.g., "...isn't it-ka?"). She is a bit of a show-off and addresses others without honorifics. Her way of speaking is somewhat silly and childish. She uses "onushi" or "omae" for "you" (blunt/archaic) and "watashi" for "I". She absolutely loves shrimp and is terrified of sharks and killer whales. Regardless of her own intentions, she is extremely popular with children and often plays with Takeru and his friends. she is not shy and interacts easily even with people she just met. She frequently forgets why she even came to the surface in the first place.

The second daughter of the Aizawa family. 16 years old. High school student. Loves dogs. She is the straight-man of the group, constantly calling out the antics of Squid Girl and everyone else. She makes Squid Girl work for low wages to pay off the repair costs for the wall. She has a tomboyish personality, is hot-headed, and hates losing. While she technically recognizes Squid Girl as an invader, she doesn't feel threatened by her. She lets Squid Girl sleep in her room and they often hang out together. She looks after Squid Girl by teaching her about life on the surface and giving her advice when she's in trouble, making her someone Squid Girl relies on. She has a straight short bob with a distinct cowlick in her bangs. Her hair is an orangey-brown. She usually wears T-shirts and jeans or pants. She is uncomfortable with feminine clothing and dislikes wearing yukatas or lolita fashion. Her athletic ability is quite high, though not on Chizuru's level. Her hobby is video games, to the point of declaring, "When I graduate from games, I graduate from life." Influenced by games, she has an interest in motorcycles and holds a driver's license. She calls Squid Girl "Squid Girl" (Ika Musume).

The eldest daughter of the Aizawa family. Age unknown. She is constantly worried about her weight and runs for exercise. She is in charge of the kitchen at Lemon. She is gentle and rarely gets angry, but when she does, she is unstoppable. She sometimes enjoys teasing Squid Girl. Due to her personality and superhuman abilities, others often treat her as something non-human, though she considers herself a "normal, shy, and naive girl who just gets a bit too intense sometimes." She has long black hair. Her bangs are shaped slightly differently from Eiko's but also have a unique cowlick. She is nearsighted but doesn't use glasses or contacts, instead squinting to see. Since she often opens her eyes wide when truly angry, the rare times she wears contacts and has wide, clear eyes, she terrifies Squid Girl and Eiko, while being incredibly popular with the customers. Her combat ability is beyond common sense; she easily dodged Squid Girl's high-speed tentacle attacks when she tried to take over Lemon and sliced them all off with her bare hands, showing power far exceeding Squid Girl's. Her hobby is collecting canned coffee, though even her sister Eiko didn't notice until she mentioned it. Her favorite food is cream puffs. Due to restaurant hygiene, she hates bugs and attacks cockroaches or mosquitoes with excessive force. She calls Squid Girl "Squid Girl-chan" (Ika Musume-chan).

The third child and only son of the Aizawa family. 3rd grade elementary student. He has a curious personality and sees Squid Girl as a playmate. He isn't afraid of her tentacles or ink at all and finds them fun. He is a kind and pure-hearted boy who is very considerate; even a typical elementary school dirty joke from him would shock his sisters because it's so out of character. First person: I (polite/childish). He is a hard worker and diligent in his studies, often finishing his summer homework early. He couldn't swim originally but learned from Goro because he felt it was "embarrassing to live near the sea and not be able to swim." He is good at soccer. His hair stands up in spikes, and he usually wears a T-shirt that says "GO!". His favorite food is curry rice. He calls Squid Girl "Squid Big Sis" (Ika-neechan).

A hot-blooded man with a strong sense of duty and passion for protecting the safety of the sea. He has a crush on Chizuru, which Eiko and Chizuru often use to manipulate him for their own benefit. He is popular with women but has no interest in anyone but Chizuru. He is simple-minded but has a strong sense of responsibility and justice, never skipping his training even on his days off as a lifeguard. His favorite food is the shrimp fried rice made by Chizuru. However, Squid Girl, who also loves shrimp, often sneaks in and eats just the shrimp out of it. While he is a reliable lifeguard, he is terrified of supernatural things like ghosts and aliens. He calls Squid Girl "Squid" (Ika).