Nyaruko (ニャル子?) is the main protagonist of the anime, manga, and light novel series and the main heroine of the visual novel game. She's a alien who is sent by the Space Defence Agency to Earth to protect Mahiro Yasaka. Nyaruko is a race of alien known as a Nyarlathotep which was originally featured in Lovecraft's books as a type of monster. She is sent by an organization called the Space Defence Agency to Earth in order to protect a human named Mahiro Yasaka. Nyaruko quickly falls in love with him though which she refers to as "love at first sight". Her feelings towards Mahiro become serious with her going so far as eventually talking about marriage and a family together. She takes the form of a human girl with long silver-colored hair, but jokingly says to Mahiro that she can change to her original form if he wants her to. In order to blend in she takes the name Nyaruko Yasaka as Mahiro's cousin when staying at Mahiro's house, and attending his school. Her actual age is unknown since she always stops Mahiro whenever he becomes curious or questions her age. When it comes to education she stated to Mahiro that she graduated from one of the best universities at the top of her class, and entered the Space Defense Agency after her exam. Despite her graduation, she joins Mahiro at his school in order to get closer to him. Nyaruko is shown to be highly into gaming, and says that Earth's entertainment is well known to aliens across the universe. While in combat, Mahiro notices that she doesn't hold back when it comes to brutal habits to any hostile aliens. She mainly uses her own form of space CQC close quarter combat weapons while fighting, with her weapon of choice being "Some Kind of Unspeakable Bar Thing". While in full force mode she adorns a black and red helmet with armor that covers her entire body.
Nyaruko has long, silver-colored hair and bright green eyes.
Her most distinctive feature is a long strand of hair on top of her head, changing shape whenever she feels a strong emotion. For instance, when she thinks about her romantic relationship with Mahiro, her strand of hair often curls into a heart shape. Her hair may sometimes take other shapes, such as changing into a zig-zag shape whenever she gets angry or jealous.
She is always friendly to Mahiro, eager to pursue him, but often shows abusive and brutal habits to any hostile aliens. She is also obsessed with otaku culture. She takes the name Nyaruko Yasaka as Mahiro's cousin when staying at Mahiro's house and attending his school. Her actual age is unknown since she and Kūko always stop Mahiro whenever he becomes curious or questions her age, although she states that she has already graduated from space college. She happens to be smart and self-confident, as she graduated from one of the best universities at the top of her class, and entered the Space Defence Agency after her exam. She is also quite wealthy, capable of spending a lot on otaku-related products. As a top agent, she mainly uses space CQC to fight (note that the definition of "space CQC" is defined by the user and is not restricted to close quarter combat weapons). She claims to be one of the best fighters on Earth, as she states that her combat level is 530,000, while a normal farmer armed with a hunting rifle is 5 (though this may be a parody of Dragonball Z). Despite this, she is always unsuccessful when dodging Mahiro's fork attacks. She shows fourth wall awareness in the first episode of the second season of the anime.
It has been shown that Nyaruko can easily irritate Mahiro with her action and word she uses. It is obvious that Nyaruko has a crush on him due to the fact that she sometimes has perverted thoughts that involve Mahiro.
Ever since Nyaruko confessed to Mahiro that she was in love with him, her presence around him can sometimes fluster him. As time goes by, the two of them spent more time with each other. Mahiro began to grow attracted to Nyaruko as a woman.
- Her birthday is on April 15, 2009 (the day the first light novel was released).