Akira Tsubaki (椿 明 Tsubaki Akira) ( Voiced by: Miyu Irino) is the main male protagonist, and boyfriend of the titular mysterious girlfriend herself, Mikoto Urabe. Tsubaki is an average boy by most standards. Although 17, he has never had a girlfriend, thus does not have the interpersonal skills that are necessary for a friendly or romantic relationship between a teenage boy and girl. He is a member of the Tsubaki family, and lives with his sister and his father. The three live together in the Tsubaki home, on the outskirts of Kazamidai. His mother passed away when he was very young, leaving his sister to raise him in her place.
Tsubaki is a fairly normal high school boy and shows very few remarkable traits. Due to Urabe's intimidating scissors skills, he can be somewhat timid and indirect in nature, taking a while to ask a simple question. Often when he has been able to muster the courage to ask something, he is likely to give up without much of a fight for fear of being accosted by Urabe. He can be very romantic though, with most of his thoughts revolving around how to get closer to, or how to impress, Urabe.
He starts the series with many conventional thoughts about relationships, wanting to hold Urabe's hand, kiss her, take her picture, etc. As Urabe does not care for these conventional things, he's usually discouraged, intimidated, or shot down.
In general Tsubaki is thoughtful and kind, often putting others before himself. He cares for his family and male friends and often worries about their problems, whether they are his fault or not. He also has a jealous side and hates the idea of anyone other than himself looking at Urabe as an object of attraction. However, he can sometimes have trouble handling his lust, or his embarrassment, when other girls come on to him.
He can be quite naive as well, unable to realize that Urabe has switched places with a different girl for several days.
While he can't remember his mother, his sister reveals that he would cry whenever he saw a picture of his mother in the house. Since then, Tsubaki has had all pictures of her hidden from him.
Tsubaki is a fan of science fiction, as can be seen by the poster above his desk in his bedroom for the 2001: A Space Odyssey film. He also seems to remember his many dreams after his meeting Urabe for the first time in class, sometimes with much detail. Because of that ability, he will many times meditate on what the dreams might mean, and how the dreams may relate to his attraction to Urabe.
In the anime, Tsubaki is more accepting of Urabe's strange ways in the beginning. This can be seen when he takes Urabe's photo and when they first hold hands; in both these situations in the manga he reluctantly accepts the outcome, but in the anime he's much happier to have these moments with Urabe.
When Urabe transfers into his class, he is initially intrigued by her, particularly after one outburst of her hysterically laughing. After school one day, he wakes Urabe up from her desk, only to find a puddle of drool left behind on her desk. Absentmindedly, he takes some on his finger and tastes it, possibly because of a dream he had earlier about a flower's necture. Shortly afterwards, he falls ill. When Urabe visits him, she claims that he is suffering from "love-sickness", which he most likely got from tasting her drool and then experiencing withdrawal.
From there he acknowledges, when questioned, that he fell in love with Urabe the moment he saw her face not hidden by her hair, when he awakened her from her drooling sleep, and thus the relationship begins, with Tsubaki quickly asking Urabe to be his girlfriend. Although she accepts, he continually feels that there is too much distance between them. Although they rarely have any physical contact, the two begin a ritual where Tsubaki tastes a bit of Urabe's drool each day. The drool can transmit feelings, among other things, between the two of them.
Tsubaki is not very aggressive in his pursuit of furthering his relationship with Urabe , slightly due to his fear of Urabe's scissors. He constantly compares his relationship to his best friend Kouhei Ueno's relationship with Ayuko Oka, and will occasionally try to emulate them. Otherwise, dreams, TV shows, Ueno's friendly advice, and other events that imply love or eroticism, will usually cause him to try and get Urabe to do something with him or for him, with varying results.
He develops a taste for girls who look like Urabe, as evidenced by his collection of Urabe look-alike Japanese idol Momoka Imai's photos. At one point in their relationship, Tsubaki displays an extreme streak of jealousy and possessiveness towards Urabe, forbidding her from wearing her hair styled in twin ponytails, or twin tails, because of the extreme cuteness of it, and that her new hair style was causing boys to admire her and be attracted to her. In a counter point demonstration of jealousy and possessiveness, Urabe forbids Tsubaki from looking at or having in his possession the picture book of teen idol Momoka Imai, even asking him to show her the book, only to have Urabe to cut it to shreds in a lightning quick scissors action display.
Tsubaki's kindness and thoughtfulness has unwittingly attracted a number of love rivals during his relationship with Urabe. These are often cause for conflict within his relationship, as he grapples with his longing for girls who are more socially available to him, as compared to his pure love for the more distant Urabe.
Although Tsubaki has been keen to attempt to kiss Urabe from the start, and comes a lot closer to kissing Urabe a number of times towards the end of the manga, it's him that suggests the idea that they shouldn't kiss until they graduate. As Urabe explains it would mean the end of their 'daily routine' in favor of kissing every day, however, he decides he'd rather keep the routine while they're still at school. Urabe agrees. Nevertheless, as the story progresses, the relationship between the two deepens into genuine, pure love. The manga strongly foreshadows their souls becoming deeply intertwined, and that they will never break up, but rather will stay together and be committed to one another for life.