The manager of the Lemon beach house who has red hair. She is a strong-minded girl and constantly has to keep Squid Girl in check. Despite her frequent irritation towards Squid Girl, however, Eiko does seem to care for her more than she cares to admit. Video games are one of her main hobbies, and studying is her primary weak point.
Eiko's older sister who has blue hair and almost never shows her eyes in most situations. Whilst she has the appearance of a kind person, she possesses superhuman strength and abilities and a fearsome aura which she will direct at anyone looking to cause trouble in the beach house. She seems to be concerned about her weight. She can also scare Squid Girl letting to make her cry.
A neighborhood friend of Eiko who has a pet dog named Alex. Eiko describes her as a typical 'airhead'. She develops an obsessive crush on Squid Girl, often trying to get intimate with her at every opportunity and getting attacked in the process, which has also resulted in her becoming something of a masochist.
A lifeguard and Eiko's childhood friend who is dedicated to protecting the sea and the people on the beach. He has a crush on Chizuru, often secretly keeping pictures of her, but so far has not put up the courage to confess his feelings to her.
A surf-loving girl who works part time at the Lemon. Unlike everyone else, she sees Squid Girl as a genuine threat to humanity and fears her greatly. Squid Girl, on the other hand, enjoys the fact that there's finally someone who fears her and thus constantly teases Nagisa whenever she can. For a short period, Nagisa started dressing up as a guy in order to attract female customers and stop Squid Girl from teasing her, but she gave up after assuming she needs to be the one who recognises her as an invader.
Voiced by: Hitomi Nabatame (Japanese), Ema Kokubun (s2e4, English section), Laura Post (English)
An extraterrestrial researcher from the United States, who believes Squid Girl to be an alien and is constantly trying to get her to admit being one so she can take her to her lab for analysis and examination. She has something of a rivalry with Sanae due to their shared obsession with Ika. She is fluent in both Japanese and English.
Harris (ハリスHarisu?), Clark (クラークKurāku?) and Martin (マーティンMātin?)
Cindy's fellow researchers who studied at MIT and work in a secret laboratory on the beach with Cindy. Although they have the talent to make beneficial contributions to mankind, their desire to communicate with aliens is more important to them, thus their reputation as 'The Three Stooges' (3バカトリオSan Bakatorio?). The contraptions they develop tend to make life miserable for the main cast, however well-meaning their intentions are.
The owner of a rival beach house, The Southern Winds (南風Minamikaze?), whose real name is unknown. A rather strict manager, he hopes to capitalise on Squid Girl's popularity by making his own creepy kigurumi masks based on her likeness, filled with various gadgets that often go awry. These kigurumi are often worn by his daughter, Ayumi. He often tries to challenge Eiko so that he can get the real deal to work for his shop.
The daughter of The Southern Winds' owner. Though her beauty attracts a lot of customers, she is very shy around people, especially men. In order to face people, her father often fits her with the Squid Girl kigurumi. She occasionally works at the Lemon as a means to improve her sociability and is quite bashful around Eiko.
Voiced by:Akemi Kanda (Japanese), Erika Harlacher (English)
A gentle girl with a peaceful aura, who sometimes talks with Squid Girl, hinting that invading might not be the best thing to do as not all humans are bad. It is implied that she also comes from the sea, as she has a hat similar to Squid Girl's and makes statements that indicate she isn't human. The first syllables of her name spell out 'tako' (たこ?), meaning octopus.