|Japanese name||相沢 栄子|
|Occupation||Student (high-school age)|
|Chizuru Aizawa (older sister)
Takeru Aizawa (younger brother)
|Anime Debut||Episode 1|
|Manga Debut||Chapter 1|
|Japanese voice||Ayumi Fujimura|
|English voice||Heather Pennington (Season 1)
Kira Vincent-Davis (Season 2)
Eiko Aizawa (相沢 栄子, Aizawa Eiko) is the most prominent human character in Shinryaku! Ika Musume. She is the second-in-command at the Lemon Beach House and also attends high school outside of her job.
Eiko is a teenage girl with average height, reddish auburn hair and amber eyes.
Unlike her older sister Chizuru, Eiko has a short temper and can be impatient. She isn't very proficient academically; instead of doing homework from school she would play video games. Eiko ends up being the only one in her family to not understand English, such that it leads to an awkward English-language conversation between her and Cindy Campbell, the latter assuming Eiko understands.
Eiko is also very fond of pets, such as Alex, the dog of her best friend Sanae Nagatsuki. This is also shown by her interactions with a miniature Ika Musume that she found on the way back home (in an imaginary sequence that turned out to be Sanae's dream).
Eiko is the first human that Ika interacts with after the latter makes landfall and declares her invasion plans. Eiko does not take Ika's invasion plans seriously, especially after learning that Ika had never been on the surface before, but after seeing what Ika's tentacles can do, she makes Ika an employee of the Lemon Beach House in order to pay for the damage she caused to the wall of the restaurant. Eiko is quick to object to Ika living at her family home, but after Chizuru allows it, Eiko finds herself sharing her bedroom with Ika.
Eiko often plays the "straight man" in interactions with Ika, sometimes teasing her about being an invader whenever she comes across anything unfamiliar to her. Eiko can also be jealous of Ika at times, such as when she finds out that Ika can solve math problems easily - her pride gets in the way of asking Ika for help on math homework. Eiko often denies her own bond with Ika, seeing her as just an employee, but she is quick to object to attempts to "borrow" her, though she eventually agrees on a temporary swap with Ayumi Tokita of the Southern Winds. By the end of Season 1, Eiko becomes Ika's team-mate in volleyball, and is sad when Ika decides to return to the sea, as well as happy when she comes back. It is due to their bond that Ika is able to grow her tentacles back and save Eiko from drowning.
Eiko respects her older sister and is usually bewildered by her willingness to allow Ika Musume do so many things. Eiko also notes how Chizuru remains calm in situations where one would normally get angry. It is implied that Eiko has been on the receiving end of Chizuru's barely-seen wrath in the past.
Gorō and Eiko were friends in the past, but rarely talk to each other in the timeframe of the series. However, Eiko sometimes does tease Gorō about his crush on Chizuru, as well as his fear of ghosts.
Eiko seems content to let her younger brother play with Ika during spare time, however, when she witnesses Takeru drown and having to be saved by Gorō, she is quick to deduce that Ika was responsible for bursting his inner tube without knowing.
Sanae and Eiko have been friends since childhood. Eiko sees Sanae's crush on Ika as off-putting. Nevertheless, Eiko cares about Sanae enough that she helps rein in her friend's desire for Ika, only to end up disappointed when Sanae loses control of herself after all her efforts.
- Though out the course of two seasons, Eiko is shown to wear three different colored long-sleeve pants, the first one is light blue, the second is tan, and the third is light gray.
- Of the three pants Eiko wears, the only one she is rarely shown with footwear is her tan one, so far only seen with footwear in episode 3, while in episode 8, Eiko is shown barefooted, and in episode 9 her foot is hidden.
- In season 2, Eiko is shown to no longer wear her tan pants, which could be considered ironic.
- Of the footwear that Eiko wears, her white sneakers is worn the most, as it is shown when she's in the beach house, and during the times she's doing normal things back home.
- Whats interesting to note about Eiko is that despite the amount of times Eiko goes barefooted, she is rarely shown kneeling barefooted (as in on her knees barefooted), only showing kneeling barefooted in the first segments of two episode of the first season, those being episodes 6 and 8, while the rest of the time she's barefooted, she is shown sitting down with her legs criss-cross, and other times she's kneeling, she has socks.
- Eiko is shown to be really ticklish on her feet and sides.