Ataru Moroboshi is the central protagonist of his story, a high school student whose life is defined by a potent combination of terrible luck and an insatiable appetite for women. He was born on April 13th, a date that is considered extraordinarily unlucky as it falls on Friday the 13th, coincides with a major earthquake, and is also Butsumetsu, the most ill-omened day in the Buddhist calendar. This cosmic misfortune is a core part of his identity, and his name itself is a pun that means struck by a falling star, perfectly foreshadowing the chaos that constantly rains down upon him.

Personality-wise, Ataru is most famously known for being a relentless and unapologetic womanizer. His primary goal in life appears to be living in the center of a harem of beautiful women, and he will stop at nothing to pursue any attractive girl he sees. His standard pickup technique is famously direct and awkward: he walks up to a stranger and immediately asks for her name, address, and phone number. He is also incredibly lazy, especially when it comes to his schoolwork, and is often depicted as a slacker who would rather chase girls or eat than study. Despite his lecherous and selfish facade, Ataru possesses a surprising amount of intelligence and cunning. He is particularly adept at thinking on his feet and engineering elaborate schemes to escape from trouble or get what he wants, even if those schemes usually backfire spectacularly. Furthermore, he has a hidden chivalrous side and cannot bear to see a woman hurt, a trait that is occasionally acknowledged by the other female characters. One notable example of this is when he went on a date with the ghost of a sick girl who had admired him, showing a capacity for genuine kindness.

Ataru's primary motivation is the pursuit of his own pleasure, which he defines as a life free of commitment where he can flirt with and be adored by countless women. This motivation is the engine for nearly all of the story's conflict, as it constantly puts him at odds with the jealous and powerful alien princess, Lum, who considers herself his wife. His role in the story is that of an unwitting catalyst for chaos. Through his actions and reactions, he draws in other eccentric characters and exacerbates every situation he encounters, turning ordinary high school life into a series of absurd, intergalactic, and supernatural disasters.

His most significant relationship is, of course, with Lum. Their entire arrangement began as a misunderstanding when his victory cry in a game of tag against her people was misinterpreted by her as a marriage proposal. While Lum is deeply and genuinely in love with him, Ataru initially rejects her claim and constantly tries to escape her. Ironically, he is the one woman he never actively chases. He typically treats her like a nuisance and is frequently electrocuted as punishment for his flirting, yet he proves time and again that he cannot live without her. Whenever Lum is genuinely upset or disappears from his life, he becomes desperate and goes to great lengths to get her back, even as he resumes his lecherous ways the moment she returns. Before Lum's arrival, Ataru was in a relationship with his childhood friend, Shinobu Miyake. He genuinely liked Shinobu, but Lum's interference and his own wandering eye doomed the relationship. As Shinobu grew tired of fighting for his attention, she eventually moved on, though she and Ataru remained on good terms as friends. Ataru also has a fierce rivalry with his wealthy, handsome, and sword-wielding classmate, Shutaro Mendo. They are constantly at odds, often fighting over the attention of women, especially Lum. Mendo represents everything Ataru is not, making him a perfect foil.

In terms of character development, Ataru changes very little on the surface. He remains a lecherous, lazy, and lucky idiot until the very end. However, the narrative subtly reveals his emotional growth, specifically regarding his feelings for Lum. While he can freely tell any other woman he loves her, he is completely incapable of saying those three words to Lum, who is the only one who truly wants to hear them. His development is not about changing who he is, but about the audience slowly realizing that his denials are a front. He demonstrates his love through actions, such as crying over her, risking his life for her, and refusing to let her go when presented with a chance to be free of her forever, even if he cannot bring himself to admit it out loud.

Despite his ordinary human appearance, Ataru possesses a number of remarkable, almost superhuman, abilities. His most notable trait is his incredible endurance and tenacity; he can survive Lum's powerful electric shocks, physical beatings, and all manner of other punishments that would kill a normal person, bouncing back almost immediately like a cockroach. He is also incredibly fast, particularly when motivated by the sight of a pretty woman, allowing him to perform feats like outrunning vehicles. His signature technique is catching a swinging katana blade with his bare hands, a skill he developed to counter Mendo's frequent sword attacks. He has also demonstrated knowledge of ninja substitution techniques and a peculiar ability to exploit the weaknesses of others, such as using food to distract the alien Rei.