Mahiro Yasaka (八坂 真尋 Yasaka Mahiro?) is the main hero of the anime, manga, and light novel series and the main protagonist of the visual novel game. He's a human boy who reluctantly finds himself under the protection of Nyaruko. He tends to attack aliens by stabbing or throwing forks whenever they make him angry. He appears to attack so fast that currently no alien has been able to dodge yet. At first he dislikes aliens and wants to kick them out, but as the story progresses he starts to become more tolerant toward their actions. He is attracted to Nyaruko (in her human form), but is put off by her aggressive advances at the same time. He has also kissed her a few times, the most prominent being their on-screen kiss during the events of the final OVA, Nyaruko F. According to Nyaruko and Kuuko, Mahiro's appearance is strangely attractive to aliens, which explains why Nyaruko falls in love with him after only looking at his profile, as well as why alien criminals wanted to kidnap him.
Mahiro is a tall, handsome and average weight, young man. He has gray-bluish eye color and brown hair color.
He is a complacent and peace-loving person, which has not changed throughout the series. He is a kind person who willingly helps strangers, like in the case of Hastur/Hasuta. He is also rational and likes to keep things in order to better assess them. He is arguably the most normal character in the whole series.
He is quite smart and mentally secure, as he was able to comprehend and summarize the existence of aliens in the world for his convenient understanding. He is also extremely tolerant as a result of his wish for peace, which leades to the result of extraterrestrials living in his household. However, even he has a breaking point (which is broken most easily by Nyaruko, notably) and will get extremely angry at someone who does something he believes that they shouldn't. This was easily showcased in the arc when Ghutatan came to his home.
He has no offensive abilities to speak of when fighting aliens, instead relying on the other aliens around him to defend him. But while this is the case, the genes of the great Deity Hunter reside within him. As a result, he has inherited his mother's prowess in the use of forks as a weapon. Whenever he gets notably angry at an alien, he uses a fork to stab them wherever in the body he sees fit to stab. This leads to great comedic effect, but leaves a question open. No one has ever been able to dodge his attack, despite their enhanced strength and speed. Therefore, it is debatable as to whether or not Mahiro is one of the fastest characters in the series.
From day one, it has been shown how easily Nyaruko can irritate Mahiro. Every action and every word she uses is easily capable of throwing off track everyone in the series. However, her ditzy and clueless attitude leads her to being at the mercy of the justified actions of a fork-bearing Mahiro.
Ever since Mahiro learned that Nyaruko was in love with him and still is, her presence around him was sometimes capable of flustering him. As time went on and the two of them spent more time with each other, he began to grow attracted to her as a woman. However, he refuses to openly admit anything of the sort to anyone. Especially after the Doki-Doki Private Academy incident, he has become more aware of the fact that Nyaruko is a girl.
Kuuko and Mahiro haven't really gotten on well when they met, seeing Kuuko as an enemy from day one, but soon they put aside their differences, Kuuko even told Mahiro to back off from Nyaruko, but soon they warmed up to each other, even faking being engaged once Kunne (Kuuko's cousin) Came into the picture.