Ataru Moroboshi is the main protagonist of his story, a seventeen-year-old student attending Tomobiki High School and placed in Class 2-4. From the moment of his birth, he is marked by extraordinary misfortune, having been born on Friday the 13th during a major earthquake, a date that also coincides with Butsumetsu, the unluckiest day in the Buddhist calendar. This legendary bad luck acts as a beacon, drawing a near-constant stream of unfortunate events, supernatural phenomena, and eccentric individuals to himself and the town of Tomobiki, where he is regarded as a bane by its residents.

His most defining personality trait is an insatiable and unapologetic lecherousness. His primary goal in life is to live in the center of a harem composed of exotic and beautiful women. Whenever he encounters an attractive girl, his instinct is to immediately ask for her name, phone number, and address, a pursuit that almost always ends in rejection and physical punishment, most often in the form of a powerful electric shock. Despite appearing lazy, academically unmotivated, and often stupid, Ataru frequently displays remarkable intelligence and cunning, particularly when devising schemes to escape from Lum or manipulate situations to his advantage. Beneath his superficial perversion, he possesses a surprising degree of chivalry, once claiming that if he hurt a woman, he could not call himself a man, and he has shown genuine kindness in moments such as caring for a class caterpillar everyone else hated or going on a date with the ghost of a sick girl who admired him from afar.

Ataru’s central role in the narrative is set in motion when he is randomly selected by a computer to represent humanity in a game of tag against the alien Oni princess, Lum, to decide the fate of the Earth. His girlfriend at the time, Shinobu Miyake, promised to marry him if he won. While he was initially unable to catch the flying Lum, he managed to steal her bikini top and used it as a lure to finally grab one of her horns. In his moment of triumph, he joyously declared his intent to marry Shinobu, but Lum misinterpreted the exclamation as a marriage proposal to herself and immediately accepted, much to his horror and chagrin. This event forms the core conflict of his story, as he is now perpetually trying to escape the alien fiancée he never wanted, while she is equally determined to stay by his side.

His key relationships are defined by this chaotic engagement. His relationship with Lum is the most complex. At first, he treats her as a cold nuisance who ruins his chances with other women. However, over time, it becomes clear that she is the only woman he truly loves, even as he vehemently denies it at every opportunity. When faced with the prospect of losing her, he goes to extraordinary lengths to prevent it, such as sacrificing his own ideal future, which was a harem of all the other female characters, to protect a future where he and Lum are married. In a notable instance where he wore cursed boxing gloves that forced him to punch anyone nearby, he repeatedly put his own face in front of Lum’s to take the blows himself. Despite these acts of devotion, once the danger passes, he returns to his flirtatious ways as if nothing happened. His relationship with Shinobu, his original girlfriend, is severely strained by Lum’s arrival and eventually fades. He also has a notable rivalry with his wealthy and handsome classmate, Shutaro Mendou, who is often the recipient of Ataru’s schemes and a rival for the attention of women.

Throughout the series, Ataru shows subtle development. He slowly warms up to Lum, becoming comfortable sharing a bed with her and showing moments of deep loneliness and depression when she is absent for extended periods. In the final story arc, even when faced with the threat of losing all his memories of Lum, he refuses to give her a simple, proper declaration of love, clinging to the stubborn and perverse principle that saying it under duress would not be genuine. His final, indirect admission comes when Lum sees that he had kept an old pair of her horns, a sentimental act that speaks louder than words.

Despite his human limitations, Ataru possesses several notable, almost superhuman, abilities. His tenacity is unmatched, described as having the durability of a cockroach and the rejuvenation of a lizard, allowing him to survive Lum’s electric shocks and countless other physical punishments without slowing down. He is also exceptionally fast, often seen in his school’s track uniform, and his speed allows him to outrun jets when properly motivated by the sight of a pretty girl. He has the ability to catch a katana blade with his bare hands, a skill he developed to deal with Mendou’s sword. On several occasions, he has demonstrated knowledge of ninja techniques, including the Body Replacement Technique, and a keen understanding of human nerve points. In more fantastical settings, such as dream worlds, his power is limited only by his imagination, and he has exhibited god-like abilities, and he can even generate a powerful lust aura from his perverted energy.