Lum is considered a magical girlfriend, though significantly different from others such as Belldandy of Oh! My Goddess and Ai of Video Girl Ai. While the latter two are openly considered "ideal" women by the protagonists of their respective series, Ataru rarely indicates publicly that Lum is anything other than the opposite of ideal.
She grew up on the Oni homeworld as a precocious girl and went to school with Benten of the Oni's rivals, the Lucky Gods, Oyuki, the ice princess, and Ran. There the four of them developed a reputation for causing massive amounts of trouble. Ran often got roped into their mischief and then took all the blame, while Lum got off unharmed, leading Ran to seek revenge. In junior high, Lum, Benten, and Oyuki's reputation grew into them becoming a vicious gang. Lum later got engaged to a handsome Oni named Rei, but dumped him because of his shallowness. When she was seventeen, the Oni attempted to invade the planet Earth, an event that would change her life forever.
That day, Lum met Ataru Moroboshi for the first time. He had been randomly selected by computer to face her in a game of tag with the fate of the earth on the line. Ataru had been promised marriage by his then-girlfriend Shinobu if he could win, which spurred him to success, but upon catching Lum he declared "Now I can get married!" Lum mistook this for a proposal to herself and accepted it. Because engagements are sacred on her planet, she is determined to be married to Ataru for the rest of her life. Since then, she has forced her way into his life as his "loving wife," much to his chagrin because it compromised his ability to flirt. The relationship between Lum and Ataru has been called "a humorously exaggerated parody of a common Japanese husband and wife dynamic."
Lum is very intelligent, but very naive about life on earth, with a worldview comparable to that of a kindergartener. She does not understand why the other guys at school feel that Ataru is not good enough for her, but she cares about their well-being. She has successfully adapted herself to life on Earth thanks to her technology. She still struggles to keep her younger cousin Ten from wreaking havoc thanks to his own naiveté about human behavior, though. For the most part, however, people who look at her for the first time view her as a highly attractive schoolgirl.
Lum is well-versed in the use of technology, and carries a variety of advanced gadgets from her society with her, with the stranger gadgets hidden in her bikini top. But while her intentions in operating them are often noble, such as copying a notebook to help Ataru study for an exam, the results are rarely what Lum intends. This may be in part because she is naive regarding the customs of Earth, so she often misunderstands what people say. The best example of this is her interpretation of Ataru's victory yell after he defeated her in tag. She believed it to be an instant marriage, rather than a proposal, or even just the celebratory shout it really was.
She drinks Tabasco sauce as if it were juice and the food she cooks is violently spicy. It does not stop her from trying to prepare edible victuals, and she enjoys preparing food for Ataru, who angers her by refusing to consume it. Eating umeboshi causes her to get drunk and she dislikes garlic. Despite efforts from everybody she knows to show her the contrary, her love for Ataru remains unchanged because he has consistently proven that he genuinely cares for her well-being in spite of his idiotic facade. These isolated events have convinced Lum that Ataru is indeed the right man for her.
In the beginning of the story, she constantly hugs and kisses Ataru whether he wants it or not, often infuriating the still-caring Shinobu and the rest of the class. But by the end, Lum has calmed down considerably and simply holds onto his arm whenever they are walking somewhere. She is usually together with Ataru and does almost everything with him, be it eating lunch, going to and from school, or going to a festival. She even enrolled herself in Tomobiki High to spend more time with him, which initially caused her Darling to faint from the shock. Because of this, Lum has come to consider and treat his room as her home, though Ataru insists she sleep in the closet. Takahashi's reason for this is that she believes two high school students shouldn't be sleeping together. (In early episodes of the anime, Lum tried to get Ataru to sleep with her, but he flatly refused.)
Lum is considered the embodiement of the struggle facing modern Japanese women: how to be independent and still fulfill the traditional role of loving wife and mother. While she is very independent minded, she continues to be supportive and loving of Ataru no matter what happens throughout the entire series.
Lum is a well known and popular character in Japan. On 1982-04-02, Lum and other characters from Urusei Yatsura appeared in two public service announcements for Kansai Electric Power Company regarding being careful with kites and koinobori around electrical lines. Hirano Fumi, the voice of Lum, did the main voice overs for the commercials.
Merchandise featuring Lum continues to be popular, and her image appears on a wide variety of printed, commercial, and electronic media. Thirty years after she was introduced (in 1978), pachinko machines featuring Lum are still being made. Lum also appeared wearing a Hanshin Tigers uniform in a series of numbered collector covers of the Daily Sports (a daily sports newspaper published in Japan) from 2003-08-12 to 2003-09-15.
A parody of Lum and Ten can be seen in the street in an issue of the Futurama comic. Lum has red hair and the tiger stripes on her clothes are brown.